Document how to set channels depth

[proppy]

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.

[proppy]

I looks like https://github.com/google/xls/blob/main/xls/modules/zstd/ is the only place where they are currently used: https://github.com/google/xls/blob/00eeaf43b42c80f8533b2ff02d068508ef6afa83/xls/modules/zstd/block_dec.x#L56-L61