Clarify the int param getter documentation

Clarify the int param getter documentation

OSSL_PARAMs that are of type OSSL_PARAM_INTEGER or
OSSL_PARAM_UNSIGNED_INTEGER can be obtained using any of the functions
EVP_PKEY_get_int_param(), EVP_PKEY_get_size_t_param() or
EVP_PKEY_get_bn_param(). The former two will fail if the parameter is too
large to fit into the C variable. We clarify this in the documentation.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17445)