rev-parse: clarify documentation for the --verify option

The old version could be read to mean that the argument has to refer
to a valid object, but that is incorrect:

* the object is not necessarily read (e.g., to check for corruption)

* if the argument is a 40-digit string of hex digits, then it is
  accepted whether or not is is the name of an existing object.

So reword the explanation to be less ambiguous.

Also fix the examples involving --verify: to be sure that the argument
refers to a commit (rather than some other kind of object), the
argument has to be suffixed with "^{commit}".  This trick is not
possible in the example involving --default, so don't imply that it is
exactly the same as the previous example.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -60,8 +60,19 @@ OPTIONS
 	instead.
 
 --verify::
-	The parameter given must be usable as a single, valid
-	object name.  Otherwise barf and abort.
+	Verify that exactly one parameter is provided, and that it
+	can be turned into a raw 20-byte SHA-1 that can be used to
+	access the object database. If so, emit it to the standard
+	output; otherwise, error out.
++
+If you want to make sure that the output actually names an object in
+your object database and/or can be used as a specific type of object
+you require, you can add "^{type}" peeling operator to the parmeter.
+For example, `git rev-parse "$VAR^{commit}"` will make sure `$VAR`
+names an existing object that is a commit-ish (i.e. a commit, or an
+annotated tag that points at a commit).  To make sure that `$VAR`
+names an existing object of any type, `git rev-parse "$VAR^{object}"`
+can be used.
 
 -q::
 --quiet::
@@ -306,12 +317,12 @@ $ git rev-parse --verify HEAD
 * Print the commit object name from the revision in the $REV shell variable:
 +
 ------------
-$ git rev-parse --verify $REV
+$ git rev-parse --verify $REV^{commit}
 ------------
 +
 This will error out if $REV is empty or not a valid revision.
 
-* Same as above:
+* Similar to above:
 +
 ------------
 $ git rev-parse --default master --verify $REV