mm/damon: explain "effective quota" on kernel-doc comment

mm/damon: explain "effective quota" on kernel-doc comment

The kernel-doc comment for 'struct damos_quota' describes how "effective
quota" is calculated, but does not explain what it is.  Actually there was
an input[1] about it.  Add the explanation on the comment.

Also, fix a trivial typo on the comment block: s/empt/empty/

[1] https://github.com/damonitor/damo/issues/17#issuecomment-2497525043

Link: https://lkml.kernel.org/r/20250110185232.54907-6-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Suggested-by: Honggyu Kim <honggyu.kim@sk.com>
Cc: Yunjeong Mun <yunjeong.mun@sk.com>
Cc: Honggyu Kim <honggyu.kim@sk.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>