Document ARIA mappings for usize properties

[blinding-pixels]

This continues the documentation work from #753 by covering the usize properties discussed in #749, using the same format established in #753.

A couple of points I would particularly appreciate confirmation on:

• AccessKit uses zero-based indexes, whereas the corresponding ARIA properties use one-based indexes. This also led me to correct the PositionInSet documentation: its value must be less than SizeOfSet, rather than less than or equal to it.
• AccessKit places SizeOfSet on the container, whereas ARIA places aria-setsize on each item.

I ran:

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