patch 9.1.0155: can only get getregion() from current buffer

Problem:  can only call getregion() for current buffer
Solution: Allow to retrieve selections from different buffers
          (Shougo Matsushita)

closes: #14131

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2024 Mar 03
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 Mar 06
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -4274,11 +4274,13 @@ getreginfo([{regname}])					*getreginfo()*
 			GetRegname()->getreginfo()
 
 getregion({pos1}, {pos2} [, {opts}])			*getregion()*
-		Returns the list of strings from {pos1} to {pos2} in current
+		Returns the list of strings from {pos1} to {pos2} from a
 		buffer.
 
 		{pos1} and {pos2} must both be |List|s with four numbers.
-		See |getpos()| for the format of the list.
+		See |getpos()| for the format of the list.  It's possible
+		to specify positions from a different buffer, but please
+		note the limitations at |getregion-notes|
 
 		The optional argument {opts} is a Dict and supports the
 		following items:
@@ -4299,6 +4301,7 @@ getregion({pos1}, {pos2} [, {opts}])			*getregion()*
 		This function is useful to get text starting and ending in
 		different columns, such as a |characterwise-visual| selection.
 
+							*getregion-notes*
 		Note that:
 		- Order of {pos1} and {pos2} doesn't matter, it will always
 		  return content from the upper left position to the lower
@@ -4308,8 +4311,12 @@ getregion({pos1}, {pos2} [, {opts}])			*getregion()*
 		- If the region is blockwise and it starts or ends in the
 		  middle of a multi-cell character, it is not included but
 		  its selected part is substituted with spaces.
-		- If {pos1} or {pos2} is not current in the buffer, an empty
+		- If {pos1} and {pos2} are not in the same buffer, an empty
 		  list is returned.
+		- {pos1} and {pos2} must belong to a |bufloaded()| buffer.
+		- It is evaluated in current window context, this makes a
+		  different if a buffer is displayed in a different window and
+		  'virtualedit' or 'list' is set
 
 		Examples: >
 			:xnoremap <CR>

runtime/doc/tags

@@ -7767,6 +7767,7 @@ getqflist-examples	quickfix.txt	/*getqflist-examples*
 getreg()	builtin.txt	/*getreg()*
 getreginfo()	builtin.txt	/*getreginfo()*
 getregion()	builtin.txt	/*getregion()*
+getregion-notes	builtin.txt	/*getregion-notes*
 getregtype()	builtin.txt	/*getregtype()*
 getscript	pi_getscript.txt	/*getscript*
 getscript-autoinstall	pi_getscript.txt	/*getscript-autoinstall*