Document use of digestof

[rhagenson]

Although digestof is mentioned in the appendix keywords table:

https://github.com/ponylang/pony-tutorial/blob/16c6f8686ea0588de278dcb03e5e11bbd1eaae09/content/appendices/keywords.md#L25

It is not documented in the body of the Tutorial.

This was observed by red on the community Zulip following them looking for un(der)documented keywords.

With the Tutorial in its current state I do not have a good plan for where to document digestof that does not:

1. Distract from a cohesive section
2. Result in a throwaway mention