Perhaps long overdue, add a documentation file for filesystem-level
encryption, a.k.a. fscrypt or fs/crypto/, to the Documentation
directory.  The new file is based loosely on the latest version of the
"EXT4 Encryption Design Document (public version)" Google Doc, but with
many improvements made, including:

- Reflect the reality that it is not specific to ext4 anymore.
- More thoroughly document the design and user-visible API/behavior.
- Replace outdated information, such as the outdated explanation of how
  encrypted filenames are hashed for indexed directories and how
  encrypted filenames are presented to userspace without the key.
  (This was changed just before release.)

For now the focus is on the design and user-visible API/behavior, not on
how to add encryption support to a filesystem --- since the internal API
is still pretty messy and any standalone documentation for it would
become outdated as things get refactored over time.
Reviewed-by: NMichael Halcrow <mhalcrow@google.com>
Signed-off-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>

Those functions are currently ignored, causing references at
the documentation to be lost. Don't ignore it.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Instead of just mention the function names, use cross-references
to the kernel-doc tags where pertinent.

While not all function documentation is included here, I
double-checked that all functions mentioned there still
exists.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Use pandoc to convert documentation to ReST by calling
Documentation/sphinx/tmplcvt script.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>