docs: Support for overriding endorsed plugin implementations

[Gustl22]

DRAFT: It should may land only after the next major release, when functionality is available in stable.

Description of what this PR is changing or adding, and why:

It is now possible to override native or Dart endorsed plugin implementations. This should be reflected in the docs.

Issues fixed by this PR (if any):

flutter/flutter#80374, flutter/flutter#59657, flutter/flutter#137040 See also: https://github.com/flutter/flutter/issues/80374#issuecomment-1141565904

PRs or commits this PR depends on (if any):

Presubmit checklist

• [x] This PR is marked as draft with an explanation if not meant to land until a future stable release.
• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.