Explain all keywords in Reference

[steveklabnik]

At least the move keyword isn't explained, but they all should be.

[mahkoh]

Those names are merely reserved and don't have an assigned meaning unfortunately.

[steveklabnik]

This is not true with move, and even then, we should explicitly say which are reserved, and how the ones that are not reserved work.

[steveklabnik]

Dup of #17969, fixed by #18375