I've made this against master. It seems to apply to 1.5 as well. Let me know if I need to do a separate pull request to get it in the 1.5 docs (if it's not frozen).
Looks handy to me.
Fixed #19394 -- Added note about using auth forms with custom user model
The note contains a link to the documentation that explains how to use the
built-in auth forms with custom user models.
merged in d546259