[SSSD] [PATCH] man: Minor fixes to filter_groups description

[Nikolai Kondrashov]

Hi everyone,

I noticed one little thing was wrong with the combined
filter_users/filter_groups description on the sssd.conf(5) manpage and also
wanted to add a note WRT nested groups behavior with filter_groups which was a
bit surprising to me. The trivial patches are attached.

Nick