doc/man3/OSSL_PARAM.pod: add details about multiple elements with same key

Usually, each element in an OSSL_PARAM array will have a unique key.
However, there may be some rare cases when a responder will handle
multiple elements with the same key.  This adds a short passage
explaining this case.
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9741)

doc/man3/OSSL_PARAM.pod

@@ -60,6 +60,10 @@ I<responder>.
 
 =back
 
+Normally, the order of the an B<OSSL_PARAM> array is not relevant.
+However, if the I<responder> can handle multiple elements with the
+same key, those elements must be handled in the order they are in.
+
 =head2 B<OSSL_PARAM> fields
 
 =over 4