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`.

src/nvim/api/vim.c

@@ -1320,7 +1320,7 @@ void nvim_set_current_tabpage(Tabpage tabpage, Error *err)
 ///     line2
 ///     line3
 ///   ]], false, -1)
-/// end, { buffer = true })
+/// end, { buf = true })
 /// ```
 ///
 /// @param data  Multiline input. Lines break at LF ("\n"). May be binary (containing NUL bytes).