docs: include shell section

Changed files
+2
doc
+2
doc/functions.xml
···
<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
+
xmlns:xi="http://www.w3.org/2001/XInclude"
xml:id="chap-functions">
<title>Functions reference</title>
<para>
···
<filename>./bin/start.sh</filename> -- relative paths are supported.
</para>
</section>
+
<xi:include href="shell.section.xml" />
<section xml:id="sec-pkgs-dockerTools">
<title>pkgs.dockerTools</title>