OvmfPkg/LockBoxLib: Update the comments for API UpdateLockBox()

OvmfPkg/LockBoxLib: Update the comments for API UpdateLockBox()

The previous commit:
MdeModulePkg/SmmLockBoxLib: Support LockBox enlarge in UpdateLockBox()

adds the support to enlarge a LockBox when using the LockBoxLib API
UpdateLockBox().

This commit is to sync the API description comment of UpdateLockBox() with
its counterparts in MdeModulePkg.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>