[css-text] Cross-link letters.

Author: fantasai

css-text/Overview.bs

@@ -319,9 +319,9 @@ Languages and Typesetting</h4>
         <dd>Puts the first <a>typographic letter unit</a> of each word in titlecase;
           other characters are unaffected.</dd>
       <dt><dfn>uppercase</dfn></dt>
-        <dd>Puts all lettersin uppercase.
+        <dd>Puts all <a>letters</a> in uppercase.
       <dt><dfn>lowercase</dfn></dt>
-        <dd>Puts all letters in lowercase.</dd>
+        <dd>Puts all <a>letters</a> in lowercase.</dd>
       <dt><dfn>full-width</dfn></dt>
         <dd>Puts all <a>typographic character units</a> in fullwidth form.
           If a character does not have a corresponding fullwidth form,