[Docs] Document username regex

docs/admin/22_registration.rst

@@ -26,6 +26,16 @@ signup for everyone. A menu item will be added for public registration.
 .. warning::
    Do not enable this feature unless you intend to run a public AlekSIS instance.
 
+Before enabling registration, you should consider restricting allowed usernames.
+By default, all ASCII characters are allowed in usernames. Often, it is advisable
+to not allow special characters. This often depends on the systems that will be
+linked to AlekSIS.
+
+To restrict usernames to a certain format, a regular expression can be defined
+in the ``Regular expression for allowed usernames`` preference. For example, to
+restrict the username to lower case letters and numbers, and beginning with a number,
+the regex can be set to ``^[a-z][a-z0-9]+$`.
+
 User invitations
 ~~~~~~~~~~~~~~~~