Document (or allow?) fractional durations in the config

[ezychlagre]

Is your feature request related to a problem? Please describe.
For input plugins (like OAR, RAPL, k8s, ...), in the toml configuration file, it is possible to change the polling frequency; this is implemented by the parameter poll_interval
By default, the unit is in seconds and we specify the value follow by the unit, for example: 1s
As alumet can support higher frequency (up to 1000Hz) which is a great thing, it is not clear in the documentation how to specify 1000Hz ?
After some tests I discovered that:

1. it is not possible to specify a decimal or float value like 0.1s or 0,1s, we got an error
2. we can specify ms as unit, so for 100Hz we must specify: 10ms

Describe the solution you'd like
Explain in the user book how to change the pool_interval (format: supported number and supported units).

[TheElectronWill]

See chronotope/humantime#34

[TheElectronWill]

In progress at chronotope/humantime#56