[css-shadow-parts-1] Clarify the algorithm with better names.

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -142,24 +142,25 @@ and exposing it could allow access to elements inside closed shadow roots.
 Part element maps are affected by the addition and removal of elements
 and changes to the part name lists and part name maps of elements in the DOM.
 
-To calculate the part element map of a shadow root, |r|:
+To calculate the part element map of a shadow root, |outerRoot|:
 
 <ol>
-	<li>For each element, |el| within |r|
-	  <ol>
-		  <li>For each |name| in |el|'s part name list,
-        add |el| to |r|'s part element map
+  <li>For each element, |el| within |outerRoot|
+    <ol>
+      <li>For each |name| in |el|'s part name list,
+        add |el| to |outerRoot|'s part element map
         under the key |name|.
-      <li>If |el| is a shadow host itself, let |er| be its shadow root:
+      <li>If |el| is a shadow host itself
+      then let |innerRoot| be its shadow root:
         <ol>
-          <li>Calculate |er|'s part element map.
-          <li>For each key, |outerName|, in |r|'s part name map
+          <li>Calculate |innerRoot|'s part element map.
+          <li>For each key, |outerName|, in |el|'s part name map
             and for each token |innerName| under that key
-            look up |innerName| in |er|'s shadow part element map
+            look up |innerName| in |innerRoot|'s shadow part element map
             to get a (possibly empty) set of elements
-            and add these elements to |r|'s part element map under |outerName|
+            and add these elements to |outerRoot|'s part element map under |outerName|
         </ol>
-		</ol>
+    </ol>
 </ol>