Document the new values that can be used for expiry values where their
use makes sense:
* git reflog expire --expire=[all|never]
* git reflog expire --expire-unreachable=[all|never]
* git gc --prune=all
Other combinations aren't useful and therefore no documentation is
added (even though they are allowed):
* git gc --prune=never
is redundant with "git gc --no-prune"
* git prune --expire=all
is equivalent to providing no --expire option
* git prune --expire=never
is a NOP
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>