doc: clarify kernel-timestamp format
doc: clarify kernel-timestamp format

closes https://github.com/rsyslog/liblognorm/issues/34

aturetta/liblognorm,aturetta/liblognorm,janmejay/liblognorm,radu-gheorghe/liblognorm,aturetta/liblognorm,janmejay/liblognorm,radu-gheorghe/liblognorm