[clang-doc] Have a description for the JSON format

There should be a document somewhere describing the possible format of the JSON output. This would help as a reference when adding new constructs to HTML or to future formats that use JSON. Right now, you have to look inside `JSONGenerator.cpp` to check whether something is an object or what the properties of an object are. That makes it difficult to easily add placeholders to templates.

Start by reading JSONGenerator.cpp, which the issue identifies as the current source of information about the JSON output. Document whether each output element is an object and list its properties so the format can serve as a reference for adding template placeholders and future JSON-based formats.

Written by the indexing model from the issue text.