new_node: added missing doxygen, cleanup.

src/knot/dnssec/zone-sign.c

src/knot/updates/changesets.h

src/knot/updates/ddns.c

src/knot/updates/ddns.h

src/knot/updates/xfr-in.c

src/knot/zone/node.c

src/knot/zone/zone-diff.c

src/libknot/rrset.c

src/libknot/rrset.h