Claridy rtc content model

fix #1424
Clarify that rtc elements optionally contain `rp` before and/or after `rt elements.

sections/semantics-textlevel.include

@@ -1644,7 +1644,9 @@
     <dt><a>Contexts in which this element can be used</a>:</dt>
     <dd>As a child of a <{ruby}> element.</dd>
     <dt><a>Content model</a>:</dt>
-    <dd><a>Phrasing content</a>, <{rt}>, or <{rp}> elements.</dd>
+    <dd>
+     <p>Either <a>Phrasing content</a>, or</p>>
+      <p>An optional <{rp}> element, followed by a sequence of <{rt}> elements, optionally followed by an <{rp}> elements.</p></dd>
     <dt><a>Tag omission in text/html</a>:</dt>
     <dd>
       An <{rtc}> element's <a>end tag</a> may be omitted if the <{rtc}> element is