vim-patch:8.2.1297: when a test fails it's often not easy to see where

Problem:    When a test fails it's often not easy to see what the call stack
            is.
Solution:   Add more entries from the call stack in the exception message.
a5d0423fa1

Use docs from latest Vim.

runtime/doc/builtin.txt

@@ -2010,6 +2010,7 @@ expand({string} [, {nosuf} [, {list}]])				*expand()*
 					a function
 			<SID>		"<SNR>123_"  where "123" is the
 					current script ID  |<SID>|
+			<stack>		call stack
 			<cword>		word under the cursor
 			<cWORD>		WORD under the cursor
 			<client>	the {clientid} of the last received

runtime/doc/cmdline.txt

@@ -879,12 +879,18 @@ Note: these are typed literally, they are not special keys!
 							*:<sfile>* *<sfile>*
 	<sfile>    When executing a `:source` command, is replaced with the
 		   file name of the sourced file.  *E498*
-		   When executing a function, is replaced with:
-		   "function {function-name}[{lnum}]"
-		   function call nesting is indicated like this:
-		   "function {function-name1}[{lnum}]..{function-name2}[{lnum}]"
+		   When executing a function, is replaced with the call stack,
+		   as with <stack> (this is for backwards compatibility, using
+		   <stack> is preferred).
 		   Note that filename-modifiers are useless when <sfile> is
-		   used inside a function.
+		   not used inside a script.
+							*:<stack>* *<stack>*
+	<stack>	   is replaced with the call stack, using
+		   "function {function-name}[{lnum}]" for a function line
+		   and "script {file-name}[{lnum}]" for a script line, and
+		   ".." in between items.  E.g.:
+		   "function {function-name1}[{lnum}]..{function-name2}[{lnum}]"
+		   If there is no call stack you get error *E489* .
 							*:<slnum>* *<slnum>*
 	<slnum>	   When executing a `:source` command, is replaced with the
 		   line number.  *E842*