Write documentation for relevant protocol methods

[austinzheng]

Protocol members like count, isEmpty, etc that would reasonably be of interest to a user should be documented explicitly. This is so they show up directly in our Jazzy docs.

[austinzheng]

Context:

I think we have to look at it from the users perspective. When I go to the doc with the intention to find out how I get the length of the List, I don’t want to have to explore a labyrinth of protocols, I just want to know what to call. (Alexander Stigsen)

There are a couple of members (like count) that are defined in a superclass marked as :nodoc:, which means that Jazzy won't generate documentation for them. I will probably end up moving those methods into the public API subclass so that we can generate documentation for them. (@jpsim)

[jpsim]

Relevant jazzy issue: https://github.com/realm/jazzy/issues/190