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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: john-cai

Documentation/git-pack-refs.txt

@@ -51,8 +51,9 @@ The command by default packs all tags and refs that are already
 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::