Add more information to the help dialog about hidden MathML.

Author: dpvc

ts/a11y/explorer/KeyExplorer.ts

@@ -194,6 +194,13 @@ MathML version of the expression or its original source format,
 creating an SVG version of the expression, and viewing various other
 information.</p>
 
+<p>Finally, selecting the "Insert Hidden MathML" item from the options
+submenu will turn of MathJax's speech and Braille generation and
+instead use visually hidden MathML that some screen readers can voice,
+though support for this is not universal across all screen readers and
+operating systems.  Selecting speech or Braille generation in their
+submenus will remove the hidden MathML again.</p>
+
 <p>For more help, see the <a
 href="https://docs.mathjax.org/en/latest/basic/accessibility.html"
 targe="_blank">MathJax accessibility documentation.</a></p>