Note the UAX 31 provenance of `HORIZONTAL_WHITESPACE`

For the `WHITESPACE` grammar rule, we cite `Pattern_White_Space`.  For
`HORIZONTAL_WHITESPACE`, we hadn't cited provenance.  Let's do that.

Horizontal whitespace, in a Unicode context, is defined by UAX
31, Section 4.1, which categorizes `Pattern_White_Space` into
line endings, ignorable format controls, and horizontal space.
The horizontal space category is exactly the two characters our
grammar specifies.