We should add some docs for the tidy tool (probably a README.md in the src/tools/tidy directory then backlink from rustc-dev-guide). Specifically, things like:

• What is tidy
• Why do we have tidy and enforce its checks
• What checks does it perform
• Tidy directives (not to be confused with compiletest directives)
• Interactions between tidy and other tools like compiletest (e.g. revision checks and stuff)