doc: releases: 4.1: announce switch to video_bits_per_pixel()

The previous commit introduces video_bits_per_pixel() and converts
all invocation relying on video_pix_fmt_bpp(), 3rd-party users will
need to do the same on their code, and this is not a 100% drop-in API,
so better document it on the release notes.

Signed-off-by: Josuah Demangeon <me@josuah.net>