Runelite: Add/improve API documentation

[deathbeam]

Currently, the documentation in API is almost absent, so it is sometimes hard to understand what is going on (also, commented code with JavaDocs looks very nice).

Current runelite-api documentation
Current runelite-client documentation

Except net.runelite.client.plugins, all fields in these APIs should be documented. We probably need to start creating PRs documenting class by class (or smaller packages) to not create giant PR, so any contributions to solving at least parts of this issue are welcome.

Also, it is very good issue for first contribution to the project/testing the waters.

[Infinitay]

Are we allowed to change method names if it's _helpful_? Of course, renaming the method calls in other classes in needed.

[deathbeam]

Reopening because people are still ignoring documentation, and bunch of API methods are undocumented AGAIN.

[abextm]

I'm removing good-for-first-contribution and help-wanted from this because most of the methods that need documentation are not obvious and have no public implementation