docs: add style rule regarding initialization

Specifically, specify that each initialization should be done on a
separate line.

src/nvim/mbyte.c

@@ -1422,7 +1422,8 @@ int utf16_to_utf8(const wchar_t *utf16, int utf16len, char **utf8)
 void mb_utflen(const char *s, size_t len, size_t *codepoints, size_t *codeunits)
   FUNC_ATTR_NONNULL_ALL
 {
-  size_t count = 0, extra = 0;
+  size_t count = 0;
+  size_t extra = 0;
   size_t clen;
   for (size_t i = 0; i < len; i += clen) {
     clen = (size_t)utf_ptr2len_len(s + i, (int)(len - i));