Wikimedia Bugzilla is closed!

There should be a prefs-help-nickname added, to explain that
'yournick' => 'Signature:'
is not for the immediately following Email Preferences, but instead for '~~~~'!

So far there are
$ grep --count prefs-help MessagesEn.php
5
prefs-help's. This would be the 6th.

There even is
'tog-fancysig' => 'Treat signature as wikitext (without an automatic link)',
but no prefs-help-nickname.

Other observations:

An administrator should never do just
 array_push($wgHiddenPrefs,'nickname');
but instead use
 array_push($wgHiddenPrefs,'nickname','fancysig');
as there is no safety feature to stop fancysig from still meaninglessly
showing up independently.

And if an administrator indeed does the aforementioned
 array_push($wgHiddenPrefs,'nickname','fancysig');
then users will be stuck with the choices they had made, unable to
change them. So the administrator should consider zapping the
appropriate fields of user_properties, etc. or only setting such
$wgHiddenPrefs in the early days of the wiki, before users have a chance
to set them.