Make prefs-help-realname more realistic

The message is shown whenever $wgMaxCredits > 0, but several actions
and extensions don't give attribution based on it anyway.
Also simplify the sentence structure a bit.

Bug: 72078
Change-Id: I4ae3abd9433c485e7066da922d2f47529b00d7d4
Merge "Make prefs-help-realname more realistic"