[CMake] Question about -E cmake_progress_start ...

[Xiangyun Kong]

Hi, 

I could not find any documentation about the semantics of
the following command,

   cmake -E cmake_progress_start Makefile 1

The cmake documention says -E should only following one of
supported commands like make_directory. I do see the above 
style of usage in a lot of places, but fail to find any
documentation about this. 

Thanks for any helps.

Xiangyun