Document use of digestof

[rhagenson]

Although digestof is mentioned in the appendix keywords table:

pony-tutorial/content/appendices/keywords.md

Line 25 in 16c6f86

| digestof | create a `USize` value that summarizes the Pony object, similar to a Java object's `hashCode()` value.

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