Split Makefile clean recipe for document sets into individual lines.

Split Makefile clean recipe for document sets into individual lines.

This is needed for less capable platforms with limits on the size of
command line argument lists.

Fixes #14732

CLA: The author has the permission to grant the OpenSSL Team the right to use this change.

Signed-off-by: Randall S. Becker <rsbecker@nexbridge.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14735)