Fix ibuffer-unmark-backward synopsis (bug#35572)

* lisp/ibuffer.el (ibuffer-mode): Fix synopsis of
ibuffer-unmark-backward along with other minor copy-edits.
1 job for emacs-26 in 25 minutes and 52 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #1687
test

00:25:52