Improve error message when using any_instance with have_received.

Make it clearer we don't support expect / allow any instance with have_received. (FWIW we could support expect but we don't currently, if anyone wants to tackle that as a separate PR (although I vote we don't ;)))
Fixes #797.
I don't think it would require monkey patching anything, so this doc string is odd to me.  I don't want to support it, though.  I'd just say `it "fails because we don't want to support it"`.