Reference git-check-ref-format in git-branch.

Its nice to have git-check-ref-format actually get mentioned in
git-branch's documentation as the syntax of a ref name must conform
to what is described in git-check-ref-format.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-branch.txt

@@ -43,6 +43,9 @@ OPTIONS
 
 <branchname>::
 	The name of the branch to create or delete.
+	The new branch name must pass all checks defined by
+	gitlink:git-check-ref-format[1].  Some of these checks
+	may restrict the characters allowed in a branch name.
 
 <start-point>::
 	The new branch will be created with a HEAD equal to this.  It may

Documentation/git-checkout.txt

@@ -35,7 +35,10 @@ OPTIONS
 	Force a re-read of everything.
 
 -b::
-	Create a new branch and start it at <branch>.
+	Create a new branch named <new_branch> and start it at
+	<branch>.  The new branch name must pass all checks defined
+	by gitlink:git-check-ref-format[1].  Some of these checks
+	may restrict the characters allowed in a branch name.
 
 -m::
 	If you have local modifications to one or more files that