feat(lua): replace buffer with buf in vim.keymap.set/del #38360

The `buffer` option remains functional but is now undocumented.
Providing both will raise an error. Since providing `buf` was disallowed
before, there is no code that will break due to using `buffer` alongside
`buf`.

runtime/lua/vim/_meta/api.lua

@@ -2018,7 +2018,7 @@ function vim.api.nvim_parse_expression(expr, flags, highlight) end
 ---     line2
 ---     line3
 ---   ]], false, -1)
---- end, { buffer = true })
+--- end, { buf = true })
 --- ```
 ---
 --- @param data string Multiline input. Lines break at LF ("\n"). May be binary (containing NUL bytes).