vim-patch:9.1.0934: hard to view an existing buffer in the preview window (#31605)

Problem:  hard to view an existing buffer in the preview window
Solution: add the :pbuffer command (Yinzuo Jiang)

Similar as `:pedit` and `:buffer` command. `:pbuffer` edits buffer [N]
from the buffer list in the preview window.

`:pbuffer` can also open special buffer, for example terminal buffer.

closes: vim/vim#16222

a2a2fe841e

Cherry-pick Test_popup_and_previewwindow_dump() changes from patch
9.0.0625.
Cherry-pick Run_noroom_for_newwindow_test() changes from patches
8.2.0432 and 9.0.0363.

Co-authored-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>

runtime/doc/index.txt

@@ -1472,6 +1472,7 @@ tag		command		action ~
 |:ownsyntax|	:ow[nsyntax]	set new local syntax highlight for this window
 |:packadd|	:pa[ckadd]	add a plugin from 'packpath'
 |:packloadall|	:packl[oadall]	load all packages under 'packpath'
+|:pbuffer|	:pb[uffer]	edit buffer in the preview window
 |:pclose|	:pc[lose]	close preview window
 |:pedit|	:ped[it]	edit file in the preview window
 |:perl|		:pe[rl]		execute perl command

runtime/doc/windows.txt

@@ -978,6 +978,13 @@ CTRL-W g }						*CTRL-W_g}*
 		it.  Make the new Preview window (if required) N high.  If N is
 		not given, 'previewheight' is used.
 
+							*:pb* *:pbuffer*
+:[N]pb[uffer][!] [+cmd] [N]
+		Edit buffer [N] from the buffer list in the preview window.
+		If [N] is not given, the current buffer remains being edited.
+		See |:buffer-!| for [!].  This will also edit a buffer that is
+		not in the buffer list, without setting the 'buflisted' flag.
+
 							*:ped* *:pedit*
 :ped[it][!] [++opt] [+cmd] {file}
 		Edit {file} in the preview window.  The preview window is