docs/HowToUseLWIP: Add an explanation how to control flags and pbuf
docs/HowToUseLWIP: Add an explanation how to control flags and pbuf

- Add an explanation how to control flags depending on driver's status, and
how to manage pbuf for input/output.

jsdosa/TizenRT,an4967/TizenRT,pillip8282/TizenRT,davidfather/TizenRT,jeongarmy/TizenRT,pillip8282/TizenRT,junmin-kim/TizenRT,jeongarmy/TizenRT,sunghan-chang/TizenRT,jeongarmy/TizenRT,chanijjani/TizenRT,jeongchanKim/TizenRT,chanijjani/TizenRT,chanijjani/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,jeongchanKim/TizenRT,sunghan-chang/TizenRT,Samsung/TizenRT,jeongarmy/TizenRT,Samsung/TizenRT,junmin-kim/TizenRT,davidfather/TizenRT,Samsung/TizenRT,an4967/TizenRT,an4967/TizenRT,jeongchanKim/TizenRT,an4967/TizenRT,pillip8282/TizenRT,pillip8282/TizenRT,jeongchanKim/TizenRT,chanijjani/TizenRT,an4967/TizenRT,sunghan-chang/TizenRT,jeongarmy/TizenRT,davidfather/TizenRT,junmin-kim/TizenRT,jsdosa/TizenRT,jeongchanKim/TizenRT,Samsung/TizenRT,Samsung/TizenRT,junmin-kim/TizenRT,jeongchanKim/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,jsdosa/TizenRT,davidfather/TizenRT,jeongarmy/TizenRT,jsdosa/TizenRT,pillip8282/TizenRT,chanijjani/TizenRT,chanijjani/TizenRT,jeongchanKim/TizenRT,davidfather/TizenRT,chanijjani/TizenRT,davidfather/TizenRT,pillip8282/TizenRT,jeongarmy/TizenRT,sunghan-chang/TizenRT,davidfather/TizenRT,an4967/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,an4967/TizenRT,pillip8282/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,sunghan-chang/TizenRT