Clarify the note about reentrant parsing in document.write()

Closes #1603. Discussion is ongoing about how custom elements and
document.write() interact, in both #1630 and in
WICG/webcomponents#537, but this at least
clears up the source of confusion in #1603.

source

@@ -89980,7 +89980,9 @@ document.body.appendChild(frame)</pre>
     <p class="note">If the <code data-x="dom-document-write">document.write()</code> method was
     called from script executing inline (i.e. executing because the parser parsed a set of
     <code>script</code> tags), then this is a <a href="#nestedParsing">reentrant invocation of the
-    parser</a>.</p>
+    parser</a>. If the <span>parser pause flag</span> is set, the tokenizer will abort immediately
+    and no HTML will be parsed, per the tokenizer's <a href="#check-parser-pause-flag">parser pause
+    flag check</a>.</p>
 
    </li>
 
@@ -100788,9 +100790,9 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
   of the last start tag to have been emitted from this tokenizer, if any. If no start tag has been
   emitted from this tokenizer, then no end tag token is appropriate.</p>
 
-  <p>Before each step of the tokenizer, the user agent must first check the <span>parser pause
-  flag</span>. If it is true, then the tokenizer must abort the processing of any nested invocations
-  of the tokenizer, yielding control back to the caller.</p>
+  <p id="check-parser-pause-flag">Before each step of the tokenizer, the user agent must first check
+  the <span>parser pause flag</span>. If it is true, then the tokenizer must abort the processing of
+  any nested invocations of the tokenizer, yielding control back to the caller.</p>
 
   <p>The tokenizer state machine consists of the states defined in the following subsections.</p>