Show a note for --strict-equality for unsafe promotions and mention it in the docs

[ilevkivskyi]

This is a follow-up for #6370

We should document why type promotions are disabled with --strict-equality. We can give examples like b'abc' == 'abc' in Python 2 vs 3, and (1/3)**10 * 3**10 == 1. We can also show a note with the non-overlapping comparison error in such cases.

[ilevkivskyi]

After all we decided to not special-case promotions for --strict-equality, so this is now irrelevant.