docs: add Backward Compatibility Notes

[kenjis]

Description

• add Backward Compatibility Notes. See Fixed consistency in language strings #6414 (comment)

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide

[paulbalandan]

As a follow up PR perhaps, we should also enumerate what are not covered by the BC promise (as I believe the size is much smaller that what are covered by BC promise).

[MGatner]

😍

[kenjis]

@paulbalandan Yes. This is a beginning.