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:
http://edgeguides.rubyonrails.org/4_0_release_notes.html#active-record-deprecations

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

The comments there seem to indicate
find_by_attribute_and_other_attribute
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
somebody)
can make it more explicit in the notes if need be.