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 <fe1f30c12225d8cd094a72a37b5acbb01ee15159@intel.com>
Cc: Fu Siyuan <eef549df2036636f79f66102b4e2dc444721df8f@intel.com>
Cc: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <91f509c31e98a6df13950454fe51aea543223eaa@intel.com>
Reviewed-by: Laszlo Ersek <7437296077edea1cb64451c122c4dac071f0cfbc@redhat.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2