h2olog: add docs about [at]appdata annotations
h2olog: add docs about [at]appdata annotations

yannick/h2o,h2o/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,devnexen/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,yannick/h2o,cwyang/h2o,devnexen/h2o,h2o/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,cwyang/h2o,cwyang/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,yannick/h2o,devnexen/h2o