[Refactor] Make UmbrellaHeader#target read-only
[Doc] Fix typos

Thanks @segiddins.
[Changelog] Stylistic improvements

Thanks @segiddins.
[Spec] Fix some more typos
[Style] Use single quotes instead
[Style] Use array of string literal syntax
[Doc] Explain why the aggregate target is weak linked
[Refactor] Use || instead of explicit branching
[Style] Write UI messages in gerund