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

Close #1182

README.md

@@ -136,7 +136,11 @@ Plug 'junegunn/fzf', { 'dir': '~/.fzf', 'do': './install --all' }
 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
 ```
 
 Reload .vimrc and `:PlugInstall` to install plugins.

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*