The simpler way for users to deal with this is to explicitly set `yield_receiver_to_any_instance_implementation_blocks` to false.  Also, may users will get this for an expression like `MyClass.any_instance...`.  What do you think about this message instead?

```
In RSpec 3, `any_instance` implementation blocks will be yielded the receiving
instance as the first block argument to allow the implementation block to use
the state of the receiver.  To maintain compatibility with RSpec 3 you need to
either set rspec-mocks' `yield_receiver_to_any_instance_implementation_blocks`
config option to `false` OR set it to `true` and update your `any_instance`
implementation blocks to account for the first block argument being the receiving instance.

To set the config option, use a snippet like:

RSpec.configure do |rspec|
  rspec.mock_with :rspec do |mocks|
    mocks.yield_receiver_to_any_instance_implementation_blocks = false
  end
end
```

Also, I think this should use `RSpec.warn_deprecation` so that it honors users' deprecation_io config setting.  However, rspec-mocks defines `RSpec.deprecate` but not `RSpec.warn_deprecation`, so we'll probably have to define that in rspec-mocks.