DOCS: Mention that libcrypto has helper functions for OSSL_PARAMs

Fixes #11165
Reviewed-by: NTomas Mraz <tomas@openssl.org>
Reviewed-by: NPaul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15073)

doc/man3/OSSL_PARAM.pod

@@ -71,6 +71,12 @@ is NULL.  The usual full terminating template is:
 
 This can also be specified using L<OSSL_PARAM_END(3)>.
 
+=head2 Functional support
+
+Libcrypto offers a limited set of helper functions to handle
+B<OSSL_PARAM> items and arrays, please see L<OSSL_PARAM_get_int(3)>.
+Developers are free to extend or replace those as they see fit.
+
 =head2 B<OSSL_PARAM> fields
 
 =over 4