Proto: update comments and documentation for `max_slashing_period`

!15990 (merged) replaces the max_slashing_period protocol constant with two constants denunciation_delay and slashing_delay.

The following should be done as a follow-up of that MR:

• update comments in proto_alpha that still mention max_slashing_period
• update documentation in docs/alpha !16371 (merged)
• update changelog !16371 (merged)
• maybe: add a test that checks that 1 <= denunciation_period <= slashing_delay