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 #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: #14047
closes: #14186

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    170,
 /**/
     169,
 /**/