Update RGBDS master documentation

Author: GitHub Action

docs/gbz80.7.pdf

@@ -926,7 +926,10 @@ <h3 class="Ss" id="String_expressions"><a class="permalink" href="#String_expres
     <td><a class="permalink" href="#CHARVAL"><code class="Fn">CHARVAL</code></a>(<var class="Fa">char</var>,
       <var class="Fa">idx</var>)</td>
     <td>Returns the value at <var class="Ar">idx</var> <span class="No">of the
-      charmap entry for</span> <var class="Ar">char</var>.</td>
+      charmap entry for</span> <var class="Ar">char</var>.
+      <var class="Ar">If</var> <var class="Ar">idx</var> <span class="No">is not
+      specified,</span> <var class="Ar">char</var> <span class="No">must have a
+      single value, which is returned.</span></td>
   </tr>
 </table>
 <p class="Pp">Note that indexes count starting from 0 at the beginning, or from