patch 8.2.0017: OS/2 and MS-DOS are still mentioned

Problem:    OS/2 and MS-DOS are still mentioned, even though support was
            removed long ago.
Solution:   Update documentation. (Yegappan Lakshmanan, closes #5368)
diff --git a/runtime/doc/gui.txt b/runtime/doc/gui.txt
index 255179f..93e01ef 100644
--- a/runtime/doc/gui.txt
+++ b/runtime/doc/gui.txt
@@ -95,9 +95,7 @@
 
 Recommended place for your personal GUI initializations:
 	Unix		    $HOME/.gvimrc or $HOME/.vim/gvimrc
-	OS/2		    $HOME/.gvimrc, $HOME/vimfiles/gvimrc
-			    or $VIM/.gvimrc
-	MS-DOS and Win32    $HOME/_gvimrc, $HOME/vimfiles/gvimrc
+	Win32    	    $HOME/_gvimrc, $HOME/vimfiles/gvimrc
 			    or $VIM/_gvimrc
 	Amiga		    s:.gvimrc, home:.gvimrc, home:vimfiles:gvimrc
 			    or $VIM/.gvimrc