git-tag(1): we tag HEAD by default

The <commit>|<object> argument is actually not explained anywhere
(except implicitly in the description of an unannotated tag).  Write a
little explanation, in particular to cover the default.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-tag.txt

@@ -126,6 +126,12 @@ This option is only applicable when listing tags without annotation lines.
 	linkgit:git-check-ref-format[1].  Some of these checks
 	may restrict the characters allowed in a tag name.
 
+<commit>::
+<object>::
+	The object that the new tag will refer to, usually a commit.
+	Defaults to HEAD.
+
+
 CONFIGURATION
 -------------
 By default, 'git tag' in sign-with-default mode (-s) will use your