public-libs: Small adjustments and comments.

src/libknot/descriptor.c

src/libknot/packet/compr.c

src/libknot/packet/compr.h

src/libknot/packet/pkt.c

src/libknot/packet/pkt.h

src/libknot/packet/rrset-wire.c

src/libknot/processing/layer.c