(docs) Revise API docs for updated info, clarity, consistency, and formatting

This revision touches most of the pages in the v3 and v4 API docs, as well as
the release notes. We've:

* Standardized some squirmy terminology
* Adjusted the flow of several pages
* Caught two or three spots where the docs lagged behind the implementation
* Made the Markdown syntax a little more portable (summary: Let's not use the
  "\n: " definition list syntax anymore. Multi-graf list items and nested lists
  get indented four spaces, not two or three.)
* Added context about how certain objects work and how they relate to other objects
* Added info about how query operators interact with field data types