docs: deprecate <sfile>

runtime/lua/vim/_meta/vimfn.lua

@@ -1863,7 +1863,7 @@ function vim.fn.exp(expr) end
 ---   <abuf>    autocmd buffer number (as a String!)
 ---   <amatch>  autocmd matched name
 ---   <cexpr>    C expression under the cursor
----   <sfile>    sourced script file or function name
+---   <sfile>    deprecated, use <script> or <stack>
 ---   <slnum>    sourced script line number or function
 ---       line number
 ---   <sflnum>  script file line number, also when in