Document property value and action types

[blinding-pixels]

Hey, this covers the 12 undocumented property value types, as well as the action-related types and missing action descriptions.

Seven of the property value types have direct ARIA equivalents, which have been linked. Toggled represents either aria-checked or aria-pressed depending on the node’s role.

I ran:

• cargo fmt --all -- --check
• RUSTDOCFLAGS="-D warnings" cargo doc -p accesskit --no-deps
• cargo test -p accesskit