fscrypt: improve the "Encryption modes and usage" section

As the number of supported encryption modes has grown, the part of the
"Encryption modes and usage" section that describes the supported
encryption modes has gotten a bit messy.  It presents useful
information, but it's a bit lacking in high-level context.

Rework the section to hopefully be much more useful.

Link: https://lore.kernel.org/r/20230630064811.22569-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>