Add a deprecation warning for a function in docs

The function in question is SSL_get_peer_certificate()

CLA: trivial

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23315)

doc/man3/SSL_get_peer_certificate.pod

@@ -10,10 +10,15 @@ SSL_get1_peer_certificate - get the X509 certificate of the peer
 
  #include <openssl/ssl.h>
 
- X509 *SSL_get_peer_certificate(const SSL *ssl);
  X509 *SSL_get0_peer_certificate(const SSL *ssl);
  X509 *SSL_get1_peer_certificate(const SSL *ssl);
 
+The following function has been deprecated since OpenSSL 3.0,
+and can be hidden entirely by defining B<OPENSSL_API_COMPAT> with a suitable
+version value, see L<openssl_user_macros(7)>:
+
+ X509 *SSL_get_peer_certificate(const SSL *ssl);
+
 =head1 DESCRIPTION
 
 These functions return a pointer to the X509 certificate the