Documentation: clarify refname disambiguation rules.

Nobody should create ambiguous refs (i.e. have tag "foobar" and branch
"foobar" at the same time) that need to be disambiguated with these
rules to keep sanity, but the rules are there so document them.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-rev-parse.txt

@@ -122,14 +122,30 @@ blobs contained in a commit.
   your repository whose object name starts with dae86e.
 
 * An output from `git-describe`; i.e. a closest tag, followed by a
-  dash, a 'g', and an abbreviated object name.
+  dash, a `g`, and an abbreviated object name.
 
 * A symbolic ref name.  E.g. 'master' typically means the commit
   object referenced by $GIT_DIR/refs/heads/master.  If you
   happen to have both heads/master and tags/master, you can
   explicitly say 'heads/master' to tell git which one you mean.
+  When ambiguous, a `<name>` is disambiguated by taking the
+  first match in the following rules:
 
-* A suffix '@' followed by a date specification enclosed in a brace
+  . if `$GIT_DIR/<name>` exists, that is what you mean (this is usually
+    useful only for `HEAD`, `FETCH_HEAD` and `MERGE_HEAD`);
+
+  . otherwise, `$GIT_DIR/refs/<name>` if exists;
+
+  . otherwise, `$GIT_DIR/refs/tags/<name>` if exists;
+
+  . otherwise, `$GIT_DIR/refs/heads/<name>` if exists;
+
+  . otherwise, `$GIT_DIR/refs/remotes/<name>` if exists;
+
+  . otherwise, `$GIT_DIR/refs/remotes/<name>/HEAD` if exists.
+
+* A ref followed by the suffix '@' with a date specification
+  enclosed in a brace
   pair (e.g. '\{yesterday\}', '\{1 month 2 weeks 3 days 1 hour 1
   second ago\}' or '\{1979-02-26 18:30:00\}') to specify the value
   of the ref at a prior point in time.  This suffix may only be
@@ -146,8 +162,9 @@ blobs contained in a commit.
 * A suffix '{tilde}<n>' to a revision parameter means the commit
   object that is the <n>th generation grand-parent of the named
   commit object, following only the first parent.  I.e. rev~3 is
-  equivalent to rev{caret}{caret}{caret} which is equivalent to\
+  equivalent to rev{caret}{caret}{caret} which is equivalent to
-  rev{caret}1{caret}1{caret}1.
+  rev{caret}1{caret}1{caret}1.  See below for a illustration of
+  the usage of this form.
 
 * A suffix '{caret}' followed by an object type name enclosed in
   brace pair (e.g. `v0.99.8{caret}\{commit\}`) means the object