Update documentation for fix #10593

[rmic]

Description

Issue iterative/dvc#10593 addresses the need for a --keep flag on dvc experiments remove to keep the last n experiments instead of deleting them.
A PR has been submitted to do so. This issue goes hand in hand with it to update the corresponding documentation.

Possible documentation update

--keep : use this flag keep the experiments selected using other options (--rev, --num, or experiment names) and remove all others.