docs(firestore): add documentation for Document,Collection .on_snapshot

[HemangChothani]

Fixes: #9250

[tseaver]

Looking more closely, the Query.on_snapshot example is also incorrect: the first argument to the callback is a sorted list of keys / document paths in the updated tree. The example should illustrate how a real callback would handle all three arguments.

@crwilcox Please clarify / correct my understanding here, and suggest what a good callback example would look like.

[HemangChothani]

@tseaver conventionalcommits.org check throws linting errors for title and give suggestion to edit your pull request title to match Conventional Commit guidelines, due to that i have changed the title.