Update encoding.bs

encoding.bs

@@ -845,9 +845,12 @@ specification, excluding <a>index single-byte</a>, which have their own table:
   <td colspan=3><a href=index-gb18030-ranges.txt>index-gb18030-ranges.txt</a>
   <td>This <a>index</a> works different from all others. Listing all code points would result
   in over a million items whereas they can be represented neatly in 207 ranges combined with trivial
-  limit checks. It therefore only superficially matches the GB18030-2022 standard for code points
-  encoded as four bytes. See also <a>index gb18030 ranges code point</a> and
-  <a>index gb18030 ranges pointer</a> below.
+  limit checks. It therefore only superficially matches the GB18030-2005 standard for code points
+  encoded as four bytes. It does not match the GB18030-2022 standard as that would increase the
+  number of byte sequences mapping to Private Use code points. And the relevant Private Use code
+  points are mapped in the <a>gb18030 encoder</a> directly through a side table to maximize
+  compatibility with how they were mapped in GB18030-2005. See also
+  <a>index gb18030 ranges code point</a> and <a>index gb18030 ranges pointer</a> below.
  <tr>
   <td><dfn export>index jis0208</dfn>
   <td><a href=index-jis0208.txt>index-jis0208.txt</a>
@@ -2498,8 +2501,8 @@ consumers of content generated with <a>GBK</a>'s <a for=/>encoder</a>.
     <td>0xFE 0xA0
   </table>
 
-  <p class=note>This asymmetric encoder table is introduced to maximize compatibility with an
-  earlier iteration of <a>index gb18030</a>.
+  <p class=note>This asymmetric encoder table is introduced to maximize compatibility with
+  GB18030-2005. See also the explanation at <a>index gb18030 ranges</a>.
 
  <li><p>Let <var>pointer</var> be the <a>index pointer</a> for
  <var>code point</var> in <a>index gb18030</a>.