Documentation/module-signing.txt: bring up to date

Update the documentation to mention that ECC NIST P-384 automatic
keypair generation is available to use ECDSA signature type, in
addition to the RSA.

Drop mentions of the now removed SHA-1 and SHA-224 options.

Add the just added FIPS 202 SHA-3 module signature hashes.

Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>