Document what the stage numbers in the :$n:path syntax mean.

The git-rev-parse manpage talks about the :$n:path notation (buried deep in
a list of other syntax) but it just says $n is a "stage number" -- someone
who is not familiar with the internals of git's merge implementation is
never going to be able to figure out that "1", "2", and "3" means.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -215,7 +215,10 @@ blobs contained in a commit.
 * A colon, optionally followed by a stage number (0 to 3) and a
   colon, followed by a path; this names a blob object in the
   index at the given path.  Missing stage number (and the colon
-  that follows it) names an stage 0 entry.
+  that follows it) names an stage 0 entry. During a merge, stage
+  1 is the common ancestor, stage 2 is the target branch's version
+  (typically the current branch), and stage 3 is the version from
+  the branch being merged.
 
 Here is an illustration, by Jon Loeliger.  Both node B and C are
 a commit parents of commit node A.  Parent commits are ordered