chore: add inline documentation for what `BranchResult`s mean

[jamietanna]

Describe the proposed change(s).

Although it could be inferred by the name, it may be convenient to have more information as to what these actually mean, for folks trying to understand what it means

renovate/lib/workers/types.ts

Lines 100 to 117 in 97bdca3

	export type BranchResult =
	| 'already-existed'
	| 'automerged'
	| 'done'
	| 'error'
	| 'needs-approval'
	| 'needs-pr-approval'
	| 'not-scheduled'
	| 'no-work'
	| 'pending'
	| 'pr-created'
	| 'pr-edited'
	| 'pr-limit-reached'
	| 'commit-limit-reached'
	| 'branch-limit-reached'
	| 'rebase'
	| 'update-not-scheduled'
	| 'minimum-group-size-not-met';

[RahulGautamSingh]

Which ones do you think can be hard to understand?