Document JsonDocCk and checkmissingitems.py

[aDotInTheVoid]

The testing infra for rustdoc-json isn't well documented.

It would be good to write down how it works, as well as some tips for developing tests.

Things to documents

• jsondocck commands
• JSONPath usage
• Variables
• Semantics of jsonpath multiple selections, and when to use [*]
• no_core for smaller output
• selecting on doc to simplify queries.
• my oj format script
• only rerunning jsondocck (not x.py)
• Modifying checkmissingitems.py after changing schema

@rustbot claim

@rustbot modify labels: +A-rustdoc-json +T-rustdoc +A-contributor-roadblock

[aDotInTheVoid]

I have a draft open for this as rust-lang/rustc-dev-guide#2422