document longname parameter

doc/plugins/write.mdwn

@@ -198,6 +198,9 @@ value, then the id parameter specifies not a filename extension, but
 a whole filename that can be htmlized. This is useful for files
 like `Makefile` that have no extension.
 
+If `hook` is passed an optional "longname" parameter, this value is used
+when prompting a user to choose a page type on the edit page form.
+
 ### pagetemplate
 
 	hook(type => "pagetemplate", id => "foo", call => \&pagetemplate);