Update documentation to show how to duplicate-and-comment.

doc/commentary.txt

@@ -39,6 +39,25 @@ gcy                     Yank and then comment or uncomment [count] lines.
 {Visual}gcy             Yank and then comment or uncomment the highlighted
                         lines.
 
+You can implement a "duplicate-and-comment" operation, i.e., where text is
+yanked, commented/uncommented, and then the original text is pasted again by
+something like the following: >
+
+  function! s:yankcommentpaste(type,...)
+      if a:0
+          let [mark1, mark2] = [a:type, a:1]
+      else
+          let [mark1, mark2] = ["'[", "']"]
+      endif
+      let savereg = @"
+      execute "normal " . mark1 ."gcy" . mark2 . "]"
+      execute "normal! " . mark2 . "p" . mark1
+      let @" = savereg
+  endfunction
+  xnoremap <silent> dc     :<C-U>call<SID>yankcommentpaste("'<", "'>")<CR>
+  nnoremap <silent> dc     :<C-U>set opfunc=<SID>yankcommentpaste<CR>g@
+  nnoremap <silent> dcc    :<C-U>set opfunc=<SID>yankcommentpaste<Bar>exe 'norm! 'v:count1.'g@_'<CR>
+<
 The |User| CommentaryPost autocommand fires after a successful operation and
 can be used for advanced customization.