document new function "exists()"

source/rainerscript/functions/rs-exists.rst

@@ -0,0 +1,30 @@
+********
+exists()
+********
+
+Purpose
+=======
+
+exists($!path!varname)
+
+This function checks if the specified variable exists, in other
+words: contains a value. A variable that once was set and later
+on unset does also not exist.
+
+The function accepts a single argument, which needs to be a variable.
+It returns 0 if the variable does not exist and 1 otherwise. The
+function can be combined with any other expression to form more
+complec expressions.
+
+.. versionadded:: 8.2010.10
+
+
+Example
+=======
+
+.. code-block:: none
+
+   if exists(!$path!varname) then ...
+   if not exists($.local!var) then ...
+   if exists($!triggervar) and $msg contains "something" then ...
+