fix some inaccuracies in with_variant doc
fix some inaccuracies in with_variant doc

* the table wont create on mysql/mariadb b.c. user_name had
  no length
* "utf-8" is not recognized by mysql/mariadb, use "utf8"
* mysql or mariadb name match is determined by the URL, not the
  actual DB that is detected (I know I made it work that way but
  I forgot)
* for the 1.4 backport only, will remove the "mariadb" part
  as we dont support that API, #8408

Fixes: #8408
Change-Id: I5b0a58a3f94a3450631e2405bd07d0a77599ae26

sqlalchemy/sqlalchemy,zzzeek/sqlalchemy