gssd: Update description of "-l" option

Move most of the text in the description of the "-l" option up to
the DESCRIPTION section, to match what was done for "-n" and "-k".

The discussion is then less restricted by formatting, and we can
take the space to introduce a few concepts before describing the
behavior of rpc.gssd.

Fix a few misspellings and grammar issues while here.

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>