style: Document #[css(skip_if)]

components/style_traits/values.rs

@@ -34,6 +34,9 @@ use std::fmt::{self, Write};
 ///   dimension token, like: <member><identifier>;
 /// * if `#[css(skip)]` is found on a field, the `ToCss` call for that field
 ///   is skipped;
+/// * if `#[css(skip_if = "function")]` is found on a field, the `ToCss` call
+///   for that field is skipped if `function` returns true. This function is
+///   provided the field as an argument;
 /// * finally, one can put `#[css(derive_debug)]` on the whole type, to
 ///   implement `Debug` by a single call to `ToCss::to_css`.
 pub trait ToCss {