Now that we've re-documented the various sections, we can remove the
outdated text here and replace it with a high-level overview.

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: NEric Biggers <ebiggers@google.com>
Reviewed-by: NDominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>