DOC: Modify one example in EVP_PKEY_fromdata(3)

DOC: Modify one example in EVP_PKEY_fromdata(3)

The example to create an EC key from user data didn't show what one
could expect and application to do, especially with regard to how it's
done with raw EC functions.  We therefore refactor it to make proper
use of a BIGNUM where expected, and also use OSSL_PARAM_BLD(3) for
easier handling of the OSSL_PARAM array.

Fixes #12388

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12389)