git-gc.txt, git-reflog.txt: document new expiry options

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>

Documentation/git-gc.txt

@@ -62,8 +62,9 @@ automatic consolidation of packs.
 
 --prune=<date>::
 	Prune loose objects older than date (default is 2 weeks ago,
-	overridable by the config variable `gc.pruneExpire`).  This
-	option is on by default.
+	overridable by the config variable `gc.pruneExpire`).
+	--prune=all prunes loose objects regardless of their age.
+	--prune is on by default.
 
 --no-prune::
 	Do not prune any loose objects.

Documentation/git-reflog.txt

@@ -67,14 +67,19 @@ them.
 --expire=<time>::
 	Entries older than this time are pruned.  Without the
 	option it is taken from configuration `gc.reflogExpire`,
-	which in turn defaults to 90 days.
+	which in turn defaults to 90 days.  --expire=all prunes
+	entries regardless of their age; --expire=never turns off
+	pruning of reachable entries (but see --expire-unreachable).
 
 --expire-unreachable=<time>::
 	Entries older than this time and not reachable from
 	the current tip of the branch are pruned.  Without the
 	option it is taken from configuration
 	`gc.reflogExpireUnreachable`, which in turn defaults to
-	30 days.
+	30 days.  --expire-unreachable=all prunes unreachable
+	entries regardless of their age; --expire-unreachable=never
+	turns off early pruning of unreachable entries (but see
+	--expire).
 
 --all::
 	Instead of listing <refs> explicitly, prune all refs.