Clarifying dynamic find_by_attribute deprecations

I’ve seen a number of blog posts claiming all dynamic find_by_attribute
methods are deprecated come Rails4 (with extraction to a gem). However,
the Rails release notes specify that some of the API remains:

There was initial confusion in this pull request over what all was

The comments there seem to indicate
will be deprecated but find_by_attribute is not; the Rails release note
doesn’t seem to go that far.

I think it’s worth getting clarity on this, and then maybe I (or
can make it more explicit in the notes if need be.