Explicitly address the side-effects of plug#end() in the example

Close #1182

doc/plug.txt

@@ -172,7 +172,11 @@ Example~
     Plug '~/my-prototype-plugin'
 
     " Initialize plugin system
+    " - Automatically executes `filetype plugin indent on` and `syntax enable`.
     call plug#end()
+    " You can revert the settings after the call like so:
+    "   filetype indent off   " Disable file-type-specific indentation
+    "   syntax off            " Disable syntax highlighting
 <
                                                                   *:PlugInstall*