Improve documentation of EventContext for Firestore

[IchordeDionysos]

Related issues

https://github.com/firebase/firebase-functions/issues/739 https://github.com/firebase/firebase-functions/issues/785

Description

The functions/field documentation for EventContext, does not respect/mention Firestore in any way, which makes it confusing when reading through it.

It should mention Firestore at least in here: https://firebase.google.com/docs/reference/functions/cloud_functions.eventcontext#resource https://firebase.google.com/docs/reference/functions/cloud_functions.eventcontext#params (This is the most confusing only mentions RTDB, not Firestore)

And ideally, also explain what is sent here (maybe explicitly for Firestore as this had sparked confusion in the past): https://firebase.google.com/docs/reference/functions/cloud_functions.eventcontext#optional-auth https://firebase.google.com/docs/reference/functions/cloud_functions.eventcontext#optional-authtype

-- Also, it says in auth that it's there for RTDB and callable functions, but in the authType sections it says "all except RTDB". So authType is also null for callable functions?

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