EZP-23019: Completed BC doc

Added detailled instructions on how to fix the case where
wrong values were submitted in 5.3.[0-2].

Also moved BC doc from 5.4 to 5.3 since it gets backported,
and the bug will never exist in 5.4.
Merge pull request #1039 from ezsystems/EZP-23019-date_datetime_bc_migration

EZP-23019: Completed date & datetime BC doc