Manual.rst: add more detailed description for '... AS _exec'

extension/Documentation/Manual.rst

@@ -6170,7 +6170,29 @@ Renders images. Allows to define an alternative text and a title attribute for t
 Column: _exec
 ^^^^^^^^^^^^^
 
-Runs batch files or executables on the webserver. In case of an error, returncode and errormessage will be returned.
+Run any command on the web server.
+
+ * The command is run via web server, so with the uid of the web server.
+ * The current working directory is the current web instance (e.g. ``/var/www/html``) .
+ * All text send to 'stdout' will be returned.
+ * Text send to 'stderr' is not returned at all.
+ * If 'stderr' should be shown, redirect the output::
+
+   SELECT 'touch /root 2>&1' AS _exec
+
+ * If 'stdout' / 'stderr' should not be displayed, redirect the output::
+
+   SELECT 'touch /tmp >/dev/null' AS _exec
+   SELECT 'touch /root 2>&1 >/dev/null' AS _exec
+
+ * Multiple commands can be concatenated by `;`::
+
+   SELECT 'date; date' AS _exec
+
+ * If the return code is not 0, the string '[<rc>] ', will be prepended.
+ * If it is not wished to see the return code, just add ``true`` to fake rc of 0 (only the last rc will be reported)::
+
+   SELECT 'touch /root; true' AS _exec
 
 **Syntax**