doc: add description to dirtytime_expire_seconds

commit 1efff914 ("fs: add
dirtytime_expire_seconds sysctl") introduced dirtytime_expire_seconds
knob, but there is not description about it in
Documentation/sysctl/vm.txt.

Add the description for it.

Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>