[openssl/openssl] 75fa52: Update HMAC() documentation.

[Shane]

  Branch: refs/heads/openssl-3.0
  Home:   https://github.com/openssl/openssl
  Commit: 75fa52ddc97ba21627a2718da8376b75f250bb20
      https://github.com/openssl/openssl/commit/75fa52ddc97ba21627a2718da8376b75f250bb20
  Author: slontis <shane.lontis at oracle.com>
  Date:   2022-12-16 (Fri, 16 Dec 2022)

  Changed paths:
    M doc/man3/HMAC.pod
    M doc/man7/migration_guide.pod

  Log Message:
  -----------
  Update HMAC() documentation.

Fixes #19782

Clarify that EVP_Q_MAC() can be used as an alternative that allows
setting of the libctx.

Reviewed-by: Paul Dale <pauli at openssl.org>
Reviewed-by: Matt Caswell <matt at openssl.org>
Reviewed-by: Tomas Mraz <tomas at openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19855)

(cherry picked from commit d1ebd99397e18de367918c201c25c389f1f07005)