fscrypt: improve documentation for inline encryption

Currently the fscrypt inline encryption support is documented in the
"Implementation details" section, and it doesn't go into much detail.
It's really more than just an "implementation detail" though, as there
is a user-facing mount option.  Also, hardware-wrapped key support (an
upcoming feature) will depend on inline encryption and will affect the
on-disk format; by definition that's not just an implementation detail.

Therefore, move this documentation into its own section and expand it.

Link: https://lore.kernel.org/r/20210916174928.65529-4-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>