[CRYPTO] doc: Update api-intro.txt

This patch updates the list of transforms we support and clarifies that
the Block Ciphers interface in fact supports all ciphers including stream
ciphers.

It also removes the obsolete Configuration Notes section and adds the
linux-crypto mailing list as the primary bug reporting address.

Finally it documents the fact that setkey should only be called from
user context.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>