Doc: User Defined Command, add note on exit codes != 0

Author: Sigma1912

docs/src/gcode/m-code.adoc

@@ -696,6 +696,8 @@ parameter of the INI file) is executed with the optional P and Q values
 as its two arguments.
 
 Execution of the G-code file pauses until the external program exits.
+If the external program exits with exit code other than '0' gcode program exection is
+stopped.
 Any valid executable file can be used. The file must be located in the search
 path specified in the INI file configuration.
 See the <<sub:ini:sec:display,Display>> section for more information on search paths.