[Cdsa-doc] key generation & key length

[<mo...@ob...>]

When you generate a symmetric key, you have to give the key length
in CSSM_CSP_CreateKeyGenContext.
But the documentation is not very clear if the CSP must verify the key
size during the CSSM_CSP_CreateKeyGenContext or during
the CSSM_GenerateKey.

For example, the SSLeay CSP seems to test the key size during
CSSM_GenerateKey.
So its possible to have 2 CSP that implement the same algorithm but
which don't have the same behavior.

BR,
Nicolas