Clarify where dispatch functions/ids are defined

Clarify where dispatch functions/ids are defined

When reading the comment for ossl_dispatch_st it seems to indicate that
the function_id numbers are defined further down in the same file. But I
was not able to find them there, but instead in core_dispatch.h.

This commit suggests updating the comment to point to core_dispatch.h

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15156)