user-manual: Rewrite git-gc section for automatic packing

This should have happened back in 2007, when `git gc` learned about
auto (e9831e83, git-gc --auto: add documentation, 2007-09-17).
Signed-off-by: NW. Trevor King <wking@tremily.us>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -1561,18 +1561,12 @@ $ git stash pop
 Ensuring good performance
 -------------------------
 
-On large repositories, git depends on compression to keep the history
-information from taking up too much space on disk or in memory.
-
-This compression is not performed automatically.  Therefore you
-should occasionally run linkgit:git-gc[1]:
-
--------------------------------------------------
-$ git gc
--------------------------------------------------
-
-to recompress the archive.  This can be very time-consuming, so
-you may prefer to run `git gc` when you are not doing other work.
+On large repositories, Git depends on compression to keep the history
+information from taking up too much space on disk or in memory.  Some
+git commands may automatically run linkgit:git-gc[1], so you don't
+have to worry about running it manually.  However, compressing a large
+repository may take a while, so you may want to call `gc` explicitly
+to avoid automatic compression kicking in when it is not convenient.
 
 
 [[ensuring-reliability]]