MdeModulePkg/DxeUdpIoLib: Did some code enhancement for UdpIoLib

* Added some ASSERT descriptions for library APIs.
* Added "Optional" option for Context parameter in UdpIoCancelDgrams().
* Added function return status check for UdpIoFreeIo().

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wang Fan <fan.wang@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>