Mention what percent-encoded bytes are good for. Fixes https://www.w3…

….org/Bugs/Public/show_bug.cgi?id=21902.

url.bs

@@ -1052,8 +1052,10 @@ U+100000 to U+10FFFD.
 <a>URL parser</a>, except for code points appearing in
 <a for=url>fragments</a>.
 
-<p>The <dfn>URL units</dfn> are <a>URL code points</a> and
-<a lt="percent-encoded byte">percent-encoded bytes</a>.
+<p>The <dfn>URL units</dfn> are <a>URL code points</a> and <a>percent-encoded bytes</a>.
+
+<p class=note><a>Percent-encoded bytes</a> can be used to encode code points that are not
+<a>URL code points</a> or are excluded from a syntax production.
 
 <hr>
 
@@ -3091,6 +3093,7 @@ Joshua Bell,
 Jxck,
 Kevin Grandon,
 Larry Masinter,
+Leif Halvard Silli,
 Mark Davis,
 Marcos Cáceres,
 Martin Dürst,

url.html

@@ -1657,8 +1657,11 @@ <h3 class="heading settled" data-level="4.1" id="url-syntax"><span class="secno"
 <a data-link-type="dfn" href="#concept-url-fragment">fragments</a>.
 
 </p>
-   <p>The <dfn data-dfn-type="dfn" data-noexport="" id="url-units">URL units<a class="self-link" href="#url-units"></a></dfn> are <a data-link-type="dfn" href="#url-code-points">URL code points</a> and
-<a data-link-type="dfn" href="#percent_encoded-byte">percent-encoded bytes</a>.
+   <p>The <dfn data-dfn-type="dfn" data-noexport="" id="url-units">URL units<a class="self-link" href="#url-units"></a></dfn> are <a data-link-type="dfn" href="#url-code-points">URL code points</a> and <a data-link-type="dfn" href="#percent_encoded-byte">percent-encoded bytes</a>.
+
+</p>
+   <p class="note" role="note"><a data-link-type="dfn" href="#percent_encoded-byte">Percent-encoded bytes</a> can be used to encode code points that are not
+<a data-link-type="dfn" href="#url-code-points">URL code points</a> or are excluded from a syntax production.
 
 </p>
    <hr>
@@ -4808,6 +4811,7 @@ <h2 class="no-num heading settled" id="acknowledgments"><span class="content">Ac
 Jxck,
 Kevin Grandon,
 Larry Masinter,
+Leif Halvard Silli,
 Mark Davis,
 Marcos Cáceres,
 Martin Dürst,