(WIP) docs: improve docs for riot-rs-threads

[elenaf9]

Improve API documentation for riot-rs-threads and riot-rs-runqueue.
I'll work on next to extract the essence about scheduling in riot-rs into a markdown file in the book (most likely as User Guide and Developer Guide?) but I thought that it might make sense to already open this as a draft PR now.

Open Questions:

• Should the user/developer guide be in the book/ or docs/ folder?
• I don't fully understand Threads::flag_wait_one - @kaspar030 could you help me here?

[kaspar030]

• I don't fully understand Threads::flag_wait_one - @kaspar030 could you help me here?

Threads::flag_wait_any(mask) returns when any of mask are set, and returns all of those (set & mask). Threads::flag_wait_one(mask) returns when any of mask are set, and returns exactly one of those (ffs (set & mask)), leaving the not returned active.

This is (maybe) useful if the handler would rather handle the flags one-by-one.

[kaspar030]

• Should the user/developer guide be in the book/ or docs/ folder?

Let's have them in the book. That's markdown after all. There are sections for user/developer docs already.

[ROMemories]

Thanks for the PR! I left a few comments; some of them come from our newly introduced Coding conventions, not all code comply with them yet, but new code and refactorings should

[kaspar030]

Maybe it makes sense to split this PR into smaller chunks - maybe one per crate and one for the book?

[elenaf9]

Maybe it makes sense to split this PR into smaller chunks - maybe one per crate and one for the book?

Good idea.

Closing this PR in favor of #149, #150 and #152