I'm not sure at this stage in RHEL6's life time if there'd be much appetite for a pure documentation update - it may be better to consider this for upstream and RHEL7.

> as the option only brings in additional logs explicated in (for instance) 
> rsyslog.conf.

The option could use a better description but that's not quite accurate; the precise behaviour is plugin-defined but generally the option tells plugins that they can also collect rotated and archived logs, as well as logs in user-defined locations: all logs the plugin knows of should be included.

It is not mandatory and some plugins may behave the same regardless of --all-logs setting.

The most common pattern for implementing --all-logs is to collect all files in the log path with a glob ("/path/to/logs/*"). This is still subject to size limiting unless --log-size=0 is used (which we should also document explicitly if we're updating the man page).

For the system journal the option currently disables all scoping options (date range, line count etc.) and collects everything returned by journalctl. The journal interface available to all plugins supports these options but currently few other plugins that implement journal logs implement --all-logs for them (generally they collect the entire log by default).

It's implemented by 10 plugins in RHEL6, and about 45 in current upstream (which will soon be in RHEL7).

Some plugins also implement configuration parsing to identify additional logs (as the logs plugin does for syslog and rsyslog); by its nature this is plugin specific so we would either need to create documentation for each plugin (not a small task: there are over 220 plugins today), or extend the existing plugin description mechanism to also include some kind of summary of the log sources it will collect.