vim-patch:8.2.4726: cannot use expand() to get the script name

Problem:    Cannot use expand() to get the script name.
Solution:   Support expand('<script>'). (closes vim/vim#10121)
6013d0045d

Use `.sn_name` instead of `->sn_name` as v8.2.0154 hasn't been ported.
Cherry-pick builtin.txt expand() doc from latest Vim.

runtime/doc/builtin.txt

@@ -2013,6 +2013,8 @@ expand({string} [, {nosuf} [, {list}]])				*expand()*
 					a function
 			<SID>		"<SNR>123_"  where "123" is the
 					current script ID  |<SID>|
+			<script>	sourced script file, or script file
+					where the current function was defined
 			<stack>		call stack
 			<cword>		word under the cursor
 			<cWORD>		WORD under the cursor

runtime/doc/cmdline.txt

@@ -881,7 +881,7 @@ Note: these are typed literally, they are not special keys!
 		   file name of the sourced file.  *E498*
 		   When executing a function, is replaced with the call stack,
 		   as with <stack> (this is for backwards compatibility, using
-		   <stack> is preferred).
+		   <stack> or <script> is preferred).
 		   Note that filename-modifiers are useless when <sfile> is
 		   not used inside a script.
 							*:<stack>* *<stack>*
@@ -891,6 +891,12 @@ Note: these are typed literally, they are not special keys!
 		   ".." in between items.  E.g.:
 		   "function {function-name1}[{lnum}]..{function-name2}[{lnum}]"
 		   If there is no call stack you get error *E489* .
+							*:<script>* *<script>*
+	<script>   When executing a `:source` command, is replaced with the file
+		   name of the sourced file.  When executing a function, is
+		   replaced with the file name of the script where it is
+		   defined.
+		   If the file name cannot be determined you get error *E1274* .
 							*:<slnum>* *<slnum>*
 	<slnum>	   When executing a `:source` command, is replaced with the
 		   line number.  *E842*