Fix confusing wording in the user manual

* doc/emacs/maintaining.texi (VC Undo): Remove a potentially
inaccurate, outdated, and/or confusing sentence.  (Bug#35290)
1 job for emacs-26 in 36 minutes and 56 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #1380
test

00:36:56