Document experimental runtime API on docs.rs

[sylfn]

One can also use cargo docs-rs locally for viewing docs.

Closes #1180

[sunfishcode]

I don't believe rustix::runtime is suitable to be stabilized and publicly documented on docs.rs in its current form. The safety conditions do not document what is required to use these APIs.

If you would like to see specific functions in rustix::runtime become publicly documented and stabilized, please bring them up for discussion.

[sunfishcode]

Thinking about what rustix could do to help in this area, I've now posted these two items:

#1315 adds documentation for these functions, so that at least if someone is searching for them in the docs, they'll find more information.

I've now also filed #1314 to encourage people to post about functions in rustix::runtime that would be useful to publicly expose.