Document how to set channels depth

It doesn't look we currently document how to set channels depth.

Ideally it should be mentioned in the following pages:
- https://github.com/google/xls/blob/main/docs_src/dslx_reference.md
- https://github.com/google/xls/blob/main/docs_src/tutorials/what_is_a_proc.md
- https://github.com/google/xls/blob/main/docs_src/tutorials/how_to_use_procs.md

And featured in some of the existing proc [examples](https://github.com/google/xls/tree/main/xls/examples).