Add a blurb about $new_mail_command in the mailboxes section.

doc/manual.xml.head

@@ -4666,7 +4666,9 @@ unignore posted-to:
       <para>NeoMutt provides a number of advanced features for handling (possibly
       many) folders and new mail within them, please refer to
       <xref linkend="new-mail" />for details (including in what situations and
-      how often NeoMutt checks for new mail).</para>
+      how often NeoMutt checks for new mail).  Additionally,
+      <link linkend="new-mail-command">$new_mail_command</link> can be used
+      to run a command when new mail is detected.</para>
       <para>The
       <quote>unmailboxes</quote> command is used to remove a token from the list
       of folders which receive mail. Use