Document ARIA mappings for enum-valued properties

[blinding-pixels]

This continues the documentation work by covering the enum-valued properties.

The two differences I found are that Toggled combines the separate aria-checked and aria-pressed states based on the node's role, and AriaCurrent supports the defined enum values while ARIA also treats unrecognized non-empty tokens as true.

I ran:

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