[ADD] developer/frontend: beware orphan assets

[jdkirkwood]

Add note that orphan asset records may be left after removing/renaming via the manifest - causing bundle compilation errors.

Mainly an issue on transition from old style <xml> declarations to __manifest__.py declarations - i.e. from versions <15.0 to 15.0+.

Task: 3336371

[robodoo]

Pull request status dashboard.

[jdkirkwood]

Too specific to upgrades.