[css-conditional-3] Clarify that property declarations are in style rules; not discussing at-rule descriptors here. #5929

Author: fantasai

css-conditional-3/Overview.bs

@@ -472,7 +472,8 @@ and therefore <strong>must</strong> discard the declaration as a parse error.
 
 A CSS processor is considered to <dfn export for=CSS id="dfn-support">support</dfn>
 a declaration (consisting of a property and value) if it accepts that
-declaration (rather than discarding it as a parse error).
+declaration (rather than discarding it as a parse error)
+within a [=style rule=].
 If a processor does not implement, with a usable level of support,
 the value given,
 then it <strong>must not</strong>