[css-shadow-parts-1] Clarify part name map

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -126,8 +126,10 @@ which is an [=ordered sets=] of tokens.
 
 Each element has a <dfn export for="element">part name map</dfn>
 which is an <a>ordered map</a>,
-with keys that are tokens
-and values that are [=ordered sets=] of tokens.
+with keys that are [=strings=]
+(part names to expose to selectors outside this element)
+and values that are [=ordered sets=] of [=strings=]
+(part names that are selectable inside this element).
 
 Each <a>shadow root</a> can be thought of as having a <dfn export for="shadow root">part element map</dfn>
 with keys that are [=strings=]