Document changes in boolean attributes treatment

[mgol]

See jquery/jquery#5452

[gibson042]

The table in .attr( attributeName ) will need a new row for 4.0+, and .attr( attributeName, value ) will need updates to document the acceptability of a false value and its removal behavior which is similar to null behavior except with "aria-" attributes.

Proposed text for the latter:

Note: because ARIA attributes frequently associate behavior with "false" values that differs from attribute absence, passing false as the value for an attribute whose name starts with "aria-…" will stringify that value to "false" rather than remove the attribute. To guarantee removal of an attribute, provide null as the value or use the .removeAttr() method.

[mgol]

PR: #1254