git-describe.1: clarify that "human readable" is also git-readable

The caption uses the term "human readable", but the DESCRIPTION did
not explain this in context.

Signed-off-by: Frederick Eaton <frederik@ofb.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -18,7 +18,9 @@ The command finds the most recent tag that is reachable from a
 commit.  If the tag points to the commit, then only the tag is
 shown.  Otherwise, it suffixes the tag name with the number of
 additional commits on top of the tagged object and the
-abbreviated object name of the most recent commit.
+abbreviated object name of the most recent commit. The result
+is a "human-readable" object name which can also be used to
+identify the commit to other git commands.
 
 By default (without --all or --tags) `git describe` only shows
 annotated tags.  For more information about creating annotated tags