revisions.txt: clarify the .. and ... syntax

In <rev1>..<rev2> and <rev1>...<rev2>, if either <rev1> or <rev2> is
omitted, it defaults to 'HEAD'.  Add this detail to the document.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -239,11 +239,13 @@ To summarize:
 
 '<rev1>..<rev2>'::
 	Include commits that are reachable from <rev2> but exclude
-	those that are reachable from <rev1>.
+	those that are reachable from <rev1>.  When either <rev1> or
+	<rev2> is omitted, it defaults to 'HEAD'.
 
 '<rev1>\...<rev2>'::
 	Include commits that are reachable from either <rev1> or
-	<rev2> but exclude those that are reachable from both.
+	<rev2> but exclude those that are reachable from both.  When
+	either <rev1> or <rev2> is omitted, it defaults to 'HEAD'.
 
 '<rev>{caret}@', e.g. 'HEAD{caret}@'::
   A suffix '{caret}' followed by an at sign is the same as listing