Correct Fleet Server checkin_long_poll and checkin_timestamp config descriptions (#7451)

## Summary

This PR:
- Adds a note in the [Fleet Server
scalability](https://www.elastic.co/docs/reference/fleet/fleet-server-scalability)
doc to clarify that Elastic Agent offline detection is fixed at 5
minutes and can't be configured through Fleet Server advanced settings
- Corrects the `checkin_timestamp` and `checkin_long_poll` descriptions
in the [Advanced Fleet Server
options](https://www.elastic.co/docs/reference/fleet/fleet-server-scalability#fleet-server-configuration)
section, which wrongly implied these settings affect offline detection
- Updates the Offline agent status definition in the [Monitor Elastic
Agents](https://www.elastic.co/docs/reference/fleet/monitor-elastic-agent)
to reflect the actual behavior

Resolves elastic/docs-content-internal#1318

## Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this
contribution?
- [x] Yes  
- [ ] No  

2. If you answered "Yes" to the previous question, please specify the
tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor / Claude Opus 4.8 High

---------

Co-authored-by: Shaunak Kashyap <ycombinator@gmail.com>

Author: vishaangelova

reference/fleet/fleet-server-scalability.md

@@ -66,6 +66,10 @@ stack: ga
 
 The following advanced settings are available to fine tune your {{fleet-server}} deployment.
 
+::::{note}
+You can't configure agent offline detection through these settings. An agent is marked offline after it stops checking in with {{fleet-server}} for 5 minutes. This threshold is fixed and can't be changed.
+::::
+
 `cache`
 :   `num_counters`
 :   Size of the hash table. Best practice is to have this set to 10 times the max connections.
@@ -75,10 +79,10 @@ The following advanced settings are available to fine tune your {{fleet-server}}
 
 `server.timeouts`
 :   `checkin_timestamp`
-:   How often {{fleet-server}} updates the "last activity" field for each agent. Defaults to `30s`. In a large-scale deployment, increasing this setting might improve performance. If this setting is higher than `2m`, most agents will be shown as "offline" in the Fleet UI. For a typical setup, it’s recommended that you set this value to less than `2m`.
+:   How often {{fleet-server}} updates the **Last activity** field for each agent. Defaults to `30s`. In a large-scale deployment, increasing this setting might improve performance. To avoid agents appearing offline in {{fleet}}, keep this value well below 5 minutes.
 
 `checkin_long_poll`
-:   How long {{fleet-server}} allows a long poll request from an agent before timing out. Defaults to `5m`. In a large-scale deployment, increasing this setting might improve performance.
+:   How long {{fleet-server}} allows a long poll request from an agent before timing out. Defaults to `5m`. In a large-scale deployment, increasing this setting might improve performance. This setting controls the connection lifecycle between {{fleet-server}} and agents. It doesn't affect how quickly an agent is marked offline in {{fleet}}.
 
 `server.limits`
 :   `policy_throttle`
@@ -101,7 +105,7 @@ The following advanced settings are available to fine tune your {{fleet-server}}
 :   Burst of check-ins allowed before falling back to the rate defined by `interval`.
 
 `checkin_limit.max_body_byte_size`
-:   Maximum size in bytes of the checkin API request body. Defaults to `1048576` bytes (1 MiB). Deployments running many Synthetics monitors might need to increase this value to avoid check-in failures that cause agents to appear offline or unhealthy in the Fleet UI despite monitors executing successfully. For example:
+:   Maximum size in bytes of the checkin API request body. Defaults to `1048576` bytes (1 MiB). Deployments running many Synthetics monitors might need to increase this value to avoid check-in failures that cause agents to appear offline or unhealthy in {{fleet}} despite monitors executing successfully. For example:
 
     ```yaml
     server:

reference/fleet/monitor-elastic-agent.md

@@ -55,7 +55,7 @@ The **Agents** tab in **{{fleet}}** displays a maximum of 10,000 agents, shown o
 | **Unhealthy** | {{agent}}s have errors or are running in a degraded state. An agent will be reported as `unhealthy` as a result of a configuration problem on the host system. For example, an {{agent}} may not have the correct permissions required to run an integration that has been added to the {{agent}} policy. In this case, you may need to investigate and address the situation. |
 | **Orphaned** | For {{agent}}s enrolled in {{elastic-defend}}, the `orphaned` status indicates an error in the communication between the {{agent}} service on the host system and the endpoint security service provided by {{elastic-defend}}. On agents reported as `orphaned`, the {{elastic-defend}} integration is still running and protecting the host. |
 | **Updating** | {{agent}}s are updating the agent policy, updating the binary, or enrolling or unenrolling from {{fleet}}. |
-| **Offline** | {{agent}}s have stayed in an unhealthy status for a period of time. Offline agent’s API keys remain valid. You can still see these {{agent}}s in the {{fleet}} UI and investigate them for further diagnosis if required. |
+| **Offline** | {{agent}}s have stopped checking in with {{fleet-server}} for at least 5 minutes. This offline detection threshold is fixed at 5 minutes and can't be configured. The offline agents' API keys remain valid. You can still see these {{agent}}s in the {{fleet}} UI and investigate them for further diagnosis if required. |
 | **Inactive** | {{agent}}s have been offline for longer than the time set in your [inactivity timeout](/reference/fleet/set-inactivity-timeout.md). These {{agent}}s are valid, but have been removed from the main {{fleet}} UI. |
 | **Unenrolled** | {{agent}}s have been manually unenrolled and their API keys have been removed from the system. You can [unenroll](/reference/fleet/unenroll-elastic-agent.md) an offline {{agent}} using {{agent}} actions if you determine it’s offline and no longer valid.<br>These agents need to re-enroll in {{fleet}} to be operational again. |
 | **Uninstalled** | {{agent}}s have been successfully uninstalled and removed from the host system. |