Document the extension-destdir GUC

Author: theory

doc/src/sgml/config.sgml

@@ -10379,6 +10379,22 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-extension-destdir" xreflabel="extension_destdir">
+      <term><varname>extension_destdir</varname> (<type>string</type>)
+      <indexterm>
+       <primary><varname>extension_destdir</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        This directory is prepended to paths when loading extensions (control
+        and SQL files), and to the '$libdir' directive when loading modules
+        that back functions. This parameter can only be set at server start.
+        For more information see <xref linkend="extend-extensions-files-directory"/>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      </variablelist>
     </sect2>
    </sect1>

doc/src/sgml/extend.sgml

@@ -668,8 +668,9 @@ RETURNS anycompatible AS ...
       <listitem>
        <para>
         The directory containing the extension's <acronym>SQL</acronym> script
-        file(s).  Unless an absolute path is given, the name is relative to
-        the installation's <literal>SHAREDIR</literal> directory.  The
+        file(s). Unless an absolute path is given, the name will be used to
+        search for the extension relative to the <xref linkend="guc-extension-destdir"/>
+        and the installation's <literal>SHAREDIR</literal> directory.  The
         default behavior is equivalent to specifying
         <literal>directory = 'extension'</literal>.
        </para>