- Added Percentile aggregation tests and autogen docs
- Added a new "appendix" section to FunctionInfo. Existing Percentile docs had a final, long section with info, and we need this to leep it. We have an "detailedDescription" attribute already, but it's right after the description, and it would make it harder to read the important bits of the function (types, examples...). So I'm not reusing it.
