docs: ext4.rst: add encryption and verity to features list

Mention encryption and verity in the ext4 features list.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20191226154007.4569-1-ebiggers@kernel.org


Signed-off-by: Theodore Ts'o <tytso@mit.edu>