Improve documentation for utility functions available to custom templates

Right now, the documentation links to a raw java file, which is intimidating. We should make it clear that users are encouraged to use the available utility functions, and we should make it more obvious what functions are available.

Locate the documentation page that currently links to the raw Java file, then inspect the referenced utility functions and how custom templates use them. Done means the docs explain that these utilities are encouraged and clearly identify the available functions without requiring users to start from a raw source file.

Written by the indexing model from the issue text.