State in the UAX31 profile description that a lone _ is not an identifier

This is already what the Lexer rules block says.

Author: mattheww

src/identifiers.md

@@ -27,6 +27,8 @@ The profile used from UAX #31 is:
 * Continue := [`XID_Continue`]
 * Medial := empty
 
+with the additional constraint that a single underscore character is not an identifier.
+
 > **Note**: Identifiers starting with an underscore are typically used to indicate an identifier that is intentionally unused, and will silence the unused warning in `rustc`.
 
 Identifiers may not be a [strict] or [reserved] keyword without the `r#` prefix described below in [raw identifiers](#raw-identifiers).