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

src/nvim/globals.h

@@ -363,7 +363,7 @@ EXTERN bool sys_menu INIT( = false);
 // currently active window.
 EXTERN win_T *firstwin;              // first window
 EXTERN win_T *lastwin;               // last window
-EXTERN win_T *prevwin INIT( = NULL);  // previous window
+EXTERN win_T *prevwin INIT( = NULL);  // previous window (may equal curwin)
 #define ONE_WINDOW (firstwin == lastwin)
 #define FOR_ALL_FRAMES(frp, first_frame) \
   for ((frp) = first_frame; (frp) != NULL; (frp) = (frp)->fr_next)