git-svn: trailing slash in prefix is mandatory with --branches/-b

Make clear in the documentation that when using --branches/-b and
--prefix with 'init', the prefix must include a trailing slash.
This matches the actual behavior of git-svn, e.g.:

 $ git svn init -Ttrunk -treleases -bbranches --prefix xxx \
     http://svn.sacredchao.net/svn/quodlibet/
 --prefix='xxx' must have a trailing slash '/'
 $

This was noticed by R. Vanicat and reported through
 http://bugs.debian.org/429443

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-svn.txt

@@ -66,9 +66,10 @@ COMMANDS
 	to the names of remotes if trunk/branches/tags are
 	specified.  The prefix does not automatically include a
 	trailing slash, so be sure you include one in the
-	argument if that is what you want.  This is useful if
-	you wish to track multiple projects that share a common
-	repository.
+	argument if that is what you want.  If --branches/-b is
+	specified, the prefix must include a trailing slash.
+	Setting a prefix is useful if you wish to track multiple
+	projects that share a common repository.
 
 'fetch'::
 	Fetch unfetched revisions from the Subversion remote we are