MdeModulePkg: document workaround for EFI_RUNTIME_EVENT_ENTRY PI spec bug

MdeModulePkg: document workaround for EFI_RUNTIME_EVENT_ENTRY PI spec bug

The PI spec (v1.7) correctly specifies "EFI_RUNTIME_EVENT_ENTRY.Event" in
natural language, but the field type in the structure definition itself is
wrong -- it should be EFI_EVENT, not (EFI_EVENT*).

This spec bug is likely unfixable for compatibility reasons, and so edk2
works it around already. We should clearly document the workaround.

Functionally, this patch is a no-op.

(I've also requested a non-normative (informative) clarification for the
PI spec: <https://mantis.uefi.org/mantis/view.php?id=2017>.)

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>