ShellPkg/UefiShellLib: clarify workaround for unfixable EdkShell bug
    
    The EDK 1 Shell (available at <https://github.com/tianocore/edk-Shell>)
    has a bug in its EFI_SHELL_ENVIRONMENT2.Execute() implementation that
    edk2's UefiShellLib has no choice but to work around.
    
    Improve the explanation in the code. Also, document the implicit
    EFI_HANDLE -> (EFI_HANDLE*) conversion, which happens implicitly after
    dereferencing ParentHandle, with an explicit cast.
    
    In practice, this patch is a no-op.
    
    Cc: Jaben Carsey <jaben.carsey@intel.com>
    Cc: Ray Ni <ray.ni@intel.com>
    Cc: Zhichao Gao <zhichao.gao@intel.com>
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>