Added docs for the cache:pool:clear command

[javiereguiluz]

This fixes #7173.

[chalasr]

Should we mention that the command allows to clear multiple cache pools at once?
Otherwise 👍

[javiereguiluz]

@chalasr in docs we usually just mention commands without explaining the details (same as our policy of not documenting the arguments of the methods). But if @xabbuh and @wouterj agrees with you, I'll update this. Thanks!

[wouterj]

should be php bin/console cache:pool:clear <cache-pool-name> (this is the only Windows and Unix compatible command)

[wouterj]

We can use comments to document other usages (IIRC, we do it in other places as well):

# clears the "cache.app" pool
$ php bin/console cache:pool:clear cache.app

# clears the "cache.validation" and "cache.app" pool
$ php bin/console cache:pool:clear cache.validation cache.app

[xabbuh]

Thank you @javiereguiluz.