Document the usage of `h2o_send_inline` and the function that use it.
Document the usage of `h2o_send_inline` and the function that use it.

Based on #1896, closes #1894.

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