docs: clarify git-pack-refs --all will pack all refs

--all packs not just branch tips but anything under refs/ with the
exception of hidden refs and broken refs. Clarify this in the
documentation.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-pack-refs.txt

diff --git a/Documentation/git-pack-refs.txt b/Documentation/git-pack-refs.txt
index 154081f..22f0066 100644
--- a/Documentation/git-pack-refs.txt
+++ b/Documentation/git-pack-refs.txt
@@ -51,8 +51,9 @@
 packed, and leaves other refs
 alone.  This is because branches are expected to be actively
 developed and packing their tips does not help performance.
-This option causes branch tips to be packed as well.  Useful for
-a repository with many branches of historical interests.
+This option causes all refs to be packed as well, with the exception
+of hidden refs, broken refs, and symbolic refs. Useful for a repository
+with many branches of historical interests.
 
 --no-prune::