Wikimedia: Improve .+prenom Wikidata command

Summary:

• localisation for unisex given names in French should be « prénom épicène »
• order of the arguments is now be documented in error message

Thanks to Ash Crow to have reported and suggested those points.

As an undocumented automatic error detection, we allow to use E for épicène.

Test Plan:
Tested on Dæghrefn on #wikipedia-fr:

15:57:04 < Dereckson> .+prenom x x
15:57:06 <@Daeghrefn> Attendu : F (féminin), M (masculin), U (épicène) — e.g. .+prenom Aude F

Reviewers: Ash-Crow, dereckson

Differential Revision: http://devcentral.nasqueron.org/D409