style: Move represents_keyword to the css attributes.

Bug: 1457635
Reviewed-by: xidorn
MozReview-Commit-ID: 21yuU4h34AQ

components/style_traits/specified_value_info.rs

@@ -56,8 +56,6 @@ pub type KeywordsCollectFn<'a> = &'a mut FnMut(&[&'static str]);
 /// * `#[value_info(starts_with_keyword)]` can be used on variants to
 ///   add the name of a non-unit variant (serialized like `ToCss`) into
 ///   `collect_completion_keywords`.
-/// * `#[value_info(represents_keyword)]` can be used on fields into
-///   `collect_completion_keywords`.
 pub trait SpecifiedValueInfo {
     /// Supported CssTypes by the given value type.
     ///

components/style_traits/values.rs

@@ -37,6 +37,9 @@ use std::fmt::{self, Write};
 /// * 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;
+/// * `#[css(represents_keyword)]` can be used on bool fields in order to
+///   serialize the field name if the field is true, or nothing otherwise.  It
+///   also collects those keywords for `SpecifiedValueInfo`.
 /// * 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 {