Clean up RxFire's API documentation

[shelcindra]

Firebase SDK version: rxfire

I'd like to clean up some of the rxfire docs. Here are some things I would like to do:

• Fix format error: Function parameters not displayed.
• collectionData function name displayed incorrectly.
• Clean up examples: some are hard to follow, some have extra imports.
• Update descriptions to clarify function usage.

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[rommelpe]

Thanks for the PR on the other request, @shelcindra. You can submit a PR for this one too, and we're excited to look into it.