[llvm][docs] Update the project and runtimes lists

[DavidSpickett]

And add a note to explain which variable to prefer if the project can go in both.

[DavidSpickett]

Documentation build failure is unrelated:

Warning, treated as error:
/home/runner/work/llvm-project/llvm-project/llvm/docs/RISCVUsage.rst::Anonymous hyperlink mismatch: 1 references but 0 targets.
See "backrefs" attribute for IDs.

(though I will go an fix that later)