Add descriptions to `index.rst` about the enumeration types that will be defined in the friendly module generated by `GetModule`

[junkmd]

Related: #345 and #475

I plan to merge this PR and release it as comtypes==1.4.0 at the beginning of April. The reason for not merging this PR immediately is because there might be progress on #230, and I want to provide a period of notification for the community that the names used as aliases for ctypes.c_int will be assigned to enumerations in the next release.