RT3462: Document actions when data==NULL
RT3462: Document actions when data==NULL

If data is NULL, return the size needed to hold the
derived key.  No other API to do this, so document
the behavior.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl