gitformat-index.txt has a gap in the "mode" description?

According to gitformat-index.txt [1], "mode" is 32 bits, but we've only
documented 16 bits. I tried poking around read-cache.c, and my
impression is that other 16 are just NUL. If so, it would be worth
documenting that they're unused, especially since we documented
unused bits right in that section.

[1] https://github.com/git/git/blob/master/Documentation/gitformat-index.txt#L84