[MRG+2] MAINT warn of future behaviour change proposed in #2610

Assuming my proposal in #2610 to make PRF's `labels` parameter more functional and deprecate `pos_label` goes through at some future point, it will create a compatibility issue in the case where all of the following apply:
* `y_true` and `y_pred` contain at most two labels
* `labels` specifies exactly two labels
* `pos_label` is not None (default)
* `average` is not None (generally default).

I expect this case is rare atm, but since behaviour would change with #2610 or similar (unless `labels` changed its name), we can warn in this release (0.15) and change behaviour in 0.16 or 0.17.