[DOCS] Clarify that search timeouts behave like cancellation

[DaveCTurner]

The documentation for the global search timeout setting does not describe how strictly the timeout applies. In fact, it seems that the timeout works on a polling basis, similarly to search cancellation, and therefore a search may take substantially longer than the configured timeout, and it may even do so successfully rather than timing out.

I think it'd be helpful to add this information to this documentation to clarify the situation.

[elasticmachine]

Pinging @elastic/es-search-aggs