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.

Author: traviscross

src/whitespace.md

@@ -30,10 +30,14 @@ CR -> U+000D  // Carriage return, `'\r'`
 r[lex.whitespace.intro]
 Whitespace is any non-empty string containing only characters that have the [`Pattern_White_Space`] Unicode property.
 
+r[lex.whitespace.horizontal]
+[HORIZONTAL_WHITESPACE] is the horizontal space subset of [`Pattern_White_Space`] as categorized by [UAX #31, Section 4.1][uax31-4.1].
+
 r[lex.whitespace.token-sep]
 Rust is a "free-form" language, meaning that all forms of whitespace serve only to separate _tokens_ in the grammar, and have no semantic significance.
 
 r[lex.whitespace.replacement]
 A Rust program has identical meaning if each whitespace element is replaced with any other legal whitespace element, such as a single space character.
 
 [`Pattern_White_Space`]: https://www.unicode.org/reports/tr31/
+[uax31-4.1]: https://www.unicode.org/reports/tr31/#Whitespace_and_Syntax