fsverity: improve documentation for builtin signature support

fsverity: improve documentation for builtin signature support

fsverity builtin signatures (CONFIG_FS_VERITY_BUILTIN_SIGNATURES) aren't
the only way to do signatures with fsverity, and they have some major
limitations.  Yet, more users have tried to use them, e.g. recently by
https://github.com/ostreedev/ostree/pull/2640.  In most cases this seems
to be because users aren't sufficiently familiar with the limitations of
this feature and what the alternatives are.

Therefore, make some updates to the documentation to try to clarify the
properties of this feature and nudge users in the right direction.

Note that the Integrity Policy Enforcement (IPE) LSM, which is not yet
upstream, is planned to use the builtin signatures.  (This differs from
IMA, which uses its own signature mechanism.)  For that reason, my
earlier patch "fsverity: mark builtin signatures as deprecated"
(https://lore.kernel.org/r/20221208033548.122704-1-ebiggers@kernel.org),
which marked builtin signatures as "deprecated", was controversial.

This patch therefore stops short of marking the feature as deprecated.
I've also revised the language to focus on better explaining the feature
and what its alternatives are.

Link: https://lore.kernel.org/r/20230620041937.5809-1-ebiggers@kernel.org
Reviewed-by: Colin Walters <walters@verbum.org>
Reviewed-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>