doc: Revisit doc on [Activity|ChildWorkflow]CancellationTypes + other minor lints

[mjameswh]

What was changed

• Revamped typedocs for ActivityCancellationTypes and ChildWorkflowCancellationTypes.
• Uniformize to "Cancellation" and "Cancelled" (with two Ls) everywhere that can be changed without breaking. Note that we unfortunately have some legacy inconsistencies in our protobuf APIs, so we can't fix 100% of cases.
• Uniformize coding pattern for the enum-as-const-object-of-strings idiom
  (typedoc should be on the const object, not on the type directive, and the type
  should come after the const object definition).
• Fix some broken tsdoc @default tags (it can't be followed by {@link ...}).

[chris-olszewski]

I've been in incidents caused by canceled vs cancelled