Practical docs for using Scheduling

**This is not a stable feature in holochain core and should not be documented yet.**

Must include:
- code examples
- practical, terse explanation of how to *use*
- gotchas

* Scheduling tasks
* how to schedule a function
* No arguments received; pass state to scheduled function via source chain
* idempotent
* how to mark up a schedulable function
* infallible
* must receive and return a schedule

No file, test, or documentation entry point is named. First confirm that Scheduling is stable in Holochain core before starting; if it is, locate the relevant documentation section and cover scheduling a function, marking a function as schedulable, code examples, practical usage, and gotchas. Done means the guidance accurately reflects the stable feature and includes the listed details.

Written by the indexing model from the issue text.