[librsvg] Improve a comment

[Paolo Borelli <pborelli src gnome org>]

commit ecfdb08337eb16f5ed6d55977ab70b97ba0070ca
Author: Paolo Borelli <pborelli gnome org>
Date:   Mon Nov 11 11:58:57 2019 +0100

    Improve a comment

 rsvg_internals/src/css.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/rsvg_internals/src/css.rs b/rsvg_internals/src/css.rs
index c6032a09..b7ff5e7c 100644
--- a/rsvg_internals/src/css.rs
+++ b/rsvg_internals/src/css.rs
@@ -145,7 +145,8 @@ impl<'i> AtRuleParser<'i> for DeclParser {
     type Error = ValueErrorKind;
 }
 
-/// Dummy struct to implement cssparser::QualifiedRuleParser
+/// Dummy struct to implement cssparser::QualifiedRuleParser and
+/// cssparser::AtRuleParser
 pub struct RuleParser;
 
 /// Errors from the CSS parsing process