Document sbuf_hexdump(9) in just sbuf(9)

• Remove duplicate references to sbuf_hexdump(9) from hexdump(3). sbuf_hexdump(9) already pointed back to hexdump(3) for implementation details.
• Refer to sbuf_hexdump(9) instead of sbuf(9) for completeness

MFC after: 1 week
Reviewed by: vangyzen (earlier diff)
Sponsored by: Dell EMC Isilon
Differential Revision: D9912