Update human-id-rules.rst

Clarify position on capitalisation.

drafts/human-id-rules.rst

@@ -136,3 +136,7 @@ Capitalisation
 User IDs SHOULD be case-insensitive. This SHOULD be applied based on the 
 capitalisation rules in the CLDR dataset: http://cldr.unicode.org/
 
+This check SHOULD be applied when the user ID is created, in order to prevent
+registration with the same name and different capitalisations, e.g.
+``@foo:bar`` vs ``@Foo:bar`` vs ``@FOO:bar``.
+