vim-patch:9.1.0170: Re-allow curwin == prevwin, but document it instead

Problem:  more places exist where curwin == prevwin, and it may even be
          expected in some cases.
Solution: revert v9.1.0001, but document that it's possible instead.
          (Sean Dewar)

I've had a change of heart for the following reasons:

- A quick 'n dirty [GitHub code search](https://github.com/search?q=%2F%28winnr%5C%28%5C%29%5Cs*%3D%3D%5Cs*winnr%5C%28%5B%27%22%5D%23%5B%27%22%5D%5C%29%7Cwinnr%5C%28%5B%27%22%5D%23%5B%27%22%5D%5C%29%5Cs*%3D%3D%5Cs*winnr%5C%28%5C%29%29%2F&type=code)
  reveals some cases where it's expected in the wild.

  Particularly, it made me aware `winnr() == winnr('#')` is possible when curwin
  is changed temporarily during the evaluation of a &statusline expression item
  (`%{...}`), and is used to show something different on the statusline
  belonging to the previous window; that behaviour wasn't changed in v9.1.0001,
  but it means curwin == prevwin makes sense in some cases.

- The definition and call sites of back_to_prevwin imply some expectation that
  prevwin == wp (== curwin) is possible, as it's used to skip entering the
  prevwin in that case.

- Prior to v9.1.0001, `:wincmd p` would not beep in the case that was patched in
  v9.1.0001, but now does. That resulted in vim/vim#14047 being opened, as it affected
  the CtrlP plugin.

  I find it odd that `:wincmd p` had cases where it wouldn't beep despite doing
  nothing, but it may be preferable to keep things that way (or instead also
  beep if curwin == prevwin, if that's preferred).

- After more digging, I found cases in win_free_mem, enter_tabpage,
  aucmd_restbuf and qf_open_new_cwindow where curwin == prevwin is possible
  (many of them from autocommands). Others probably exist too, especially in
  places where curwin is changed temporarily.

fixes: vim/vim#14047
closes: vim/vim#14186

d64801e913

test/old/testdir/test_window_cmd.vim

@@ -113,64 +113,6 @@ func Test_window_quit()
   bw Xa Xb
 endfunc
 
-func Test_window_curwin_not_prevwin()
-  botright split
-  call assert_equal(2, winnr())
-  call assert_equal(1, winnr('#'))
-  quit
-  call assert_equal(1, winnr())
-  call assert_equal(0, winnr('#'))
-
-  botright split
-  botright split
-  call assert_equal(3, winnr())
-  call assert_equal(2, winnr('#'))
-  1quit
-  call assert_equal(2, winnr())
-  call assert_equal(1, winnr('#'))
-
-  botright split
-  call assert_equal(1, tabpagenr())
-  call assert_equal(3, winnr())
-  call assert_equal(2, winnr('#'))
-  wincmd T
-  call assert_equal(2, tabpagenr())
-  call assert_equal(1, winnr())
-  call assert_equal(0, winnr('#'))
-  tabfirst
-  call assert_equal(1, tabpagenr())
-  call assert_equal(2, winnr())
-  call assert_equal(0, winnr('#'))
-
-  tabonly
-  botright split
-  wincmd t
-  wincmd p
-  call assert_equal(3, winnr())
-  call assert_equal(1, winnr('#'))
-  quit
-  call assert_equal(2, winnr())
-  call assert_equal(1, winnr('#'))
-
-  botright split
-  wincmd t
-  wincmd p
-  call assert_equal(1, tabpagenr())
-  call assert_equal(3, winnr())
-  call assert_equal(1, winnr('#'))
-  wincmd T
-  call assert_equal(2, tabpagenr())
-  call assert_equal(1, winnr())
-  call assert_equal(0, winnr('#'))
-  tabfirst
-  call assert_equal(1, tabpagenr())
-  call assert_equal(2, winnr())
-  call assert_equal(1, winnr('#'))
-
-  tabonly
-  only
-endfunc
-
 func Test_window_horizontal_split()
   call assert_equal(1, winnr('$'))
   3wincmd s