Fix documentation behaviors for the new proxy functions for custom datasource fields

Summary: Ref T13090. The doc string in "any()" wasn't specified correctly and the help page wasn't getting enough supporting data to build properly.

Test Plan: Viewed "Reference: Advanced Functions" for a custom datasource field and got more helpful help.

Maniphest Tasks: T13090

Differential Revision: https://secure.phabricator.com/D19128