return resolved name, not original name

[davidism]

Reverts #1422, but updates the documentation to get the desired behavior instead. Always returning the command's original name ignored that the command could be registered with a different name like group.add_command(cmd, name).

• fixes Click 8.0.0 group.add_command(..) does not replace name in description and when looking up defaults #1895

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.