Custom Path Wildcard Constraints need Documentation

The Custom path parser allows setting constraints on what will be matched to each wildcard. This is a powerful feature, but has no public facing documentation. Updating this should be a priority in documentation improvement.

No file or test is named in the issue. Start by locating the public documentation and custom path parser, then document wildcard constraints with an example; done when users can find and understand how to configure them.

Written by the indexing model from the issue text.