[docs] Add Deprecated section to ReleaseNotes

This is brought up in https://reviews.llvm.org/D83915.
We would like to remove some feature in PowerPC.

We did send RFC before, but we think it might be a better idea that
we indicate planned removal in the Release Notes for version 11
and actual removal in those for version 12..

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D83968