[Tracker] Documentation changes

Opening this issue to track issues which are resolved by documentation changes in a single place.

https://github.com/firebase/firebase-functions/issues/1019

This is addressed in https://github.com/firebase/firebase-functions/pull/1913

https://github.com/firebase/firebase-functions/issues/1502

- Needed doc change: document the caveat that `setGlobalOptions()` must run before function definitions are evaluated, and that with multi-file JS/ESM layouts, import order / module evaluation can prevent global options from applying as users expect.
- Why: the current 2nd gen reference still states that `setGlobalOptions()` “sets default options for all functions written using the 2nd gen SDK,” but the docs do not mention the multi-file / ESM import-order caveat or recommend a safe pattern. The organize/manage pages also do not describe this.
- Docs checked: [2nd gen API reference](https://firebase.google.com/docs/reference/functions/2nd-gen/node/firebase-functions), [Manage functions](https://firebase.google.com/docs/functions/manage-functions), [Organize multiple functions](https://firebase.google.com/docs/functions/organize-functions)

https://github.com/firebase/firebase-functions/issues/1616

The documentation regarding the process of upgrading from v1 to v2 has been updated here to make things clearer.