NetworkPkg/UefiPxeBcDxe: Add the clarification compared to UefiPxeBcDxe in MdeModulePkg.

NetworkPkg/UefiPxeBcDxe: Add the clarification compared to UefiPxeBcDxe in MdeModulePkg.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1205

This patch is to add the driver usage/difference clarification
compared to UefiPxeBcDxe in MdeModulePkg.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>