Cleanup the .val() docs, remove nonstandard selectors

[mgol]

The .val() page needs cleanup with regards to some pseudo-classes. It was originally reported by @gibson042 in #981 (comment):

I (...) would (...) like to see some (...) cleanup there:

Replace all use of nonstandard :selected with standard :checked
Replace nonstandard :checkbox with [type=checkbox]
Replace nonstandard :radio with [type=radio]