Expose mbedtls_ssl_get_hostname_pointer()

In 2.28, the `hostname` field of `mbedtls_ssl_context` is part of the public
API. We've slightly changed its meaning in order to fix a security issue.
Document the new function mbedtls_ssl_get_hostname_pointer() which
returns what used to be the value of this field.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 541fcc8..f89f470 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1449,7 +1449,10 @@
      *  \p hostname argument.
      * - A special value to indicate that mbedtls_ssl_set_hostname()
      *   was called with \p NULL (as opposed to never having been called).
-     *   See `mbedtls_ssl_get_hostname_pointer()` in `ssl_tls.c`.
+     *
+     * If you need to obtain the value passed to
+     * mbedtls_ssl_set_hostname() even if it may have been called with
+     * \p NULL, call mbedtls_ssl_get_hostname_pointer().
      *
      * If this field contains the value \p NULL and the configuration option
      * #MBEDTLS_SSL_CLI_ALLOW_WEAK_CERTIFICATE_VERIFICATION_WITHOUT_HOSTNAME