[APM] Clarify TBS storage limit indexing destination and TTL expiry behavior

[alexandra5000]

Summary

Addresses two customer questions about tail-based sampling (TBS) behavior:

• Storage limit: Clarifies that when the storage limit is reached and discard_on_write_failure is false (default), events bypass TBS and are indexed immediately to the traces-apm-* data streams.
• TTL: Clarifies that trace events whose TTL expires before a sampling decision is made are silently dropped and never indexed.

Sourced from dev clarification in elastic/sdh-apm#1928.

Closes #631

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• 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: Claude Sonnet 4.6, running inside Claude Code to double-check files to be updated in the repo + write PR description

[alexandra5000]

Hey @marclop! Tagging you for review, since you were responding in the SDH issue. It's a minor change to the docs, would be great if you could take a look and double-check it.

[marclop]

Thank you!

[vishaangelova]

LGTM.