add note about genwrapper code running before wrapper sanitizes environment

(That is needed for the receive test code.)

doc/plugins/write.mdwn

@@ -597,6 +597,9 @@ describes the plugin as a whole. For example:
 This hook is used to inject C code (which it returns) into the `main`
 function of the ikiwiki wrapper when it is being generated. 
 
+The code runs before anything else -- in particular it runs before
+the suid wrapper has sanitized its environment.
+
 ## Exported variables
 
 Several variables are exported to your plugin when you `use IkiWiki;`