When keyctl_read() is passed a buffer that is too small, the behavior is
inconsistent.  Some key types will fill as much of the buffer as
possible, while others won't copy anything.  Moreover, the in-kernel
documentation contradicted the man page on this point.

Update the in-kernel documentation to say that this point is
unspecified.
Signed-off-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>