pt-stalk: Mention using $prefix in plugins

bin/pt-stalk

@@ -1936,6 +1936,10 @@ Plugins can stop the tool by setting the global variable C<OKTORUN>
 to C<1>.  In this case, the global variable C<EXIT_REASON> should also
 be set to indicate why the tool was stopped.
 
+Plugin writers should keep in mind that the file destination prefix currently
+in use should be accessed through the C<$prefix> variable, rather than
+C<$OPT_PREFIX>.
+
 =item --port
 
 short form: -P; type: int