Valid properties for get_cmake_property() no longer documented

The documentation for the get_cmake_property() command changed between CMake 3.3 and 3.4. In that change, all the valid property names were removed and there's nowhere else in the CMake documentation which provides this information. As a result, it's technically a complete mystery what property names one can use with get_cmake_property() if you're going only by the latest documentation.

As it turns out, get_cmake_property(myVar COMPONENTS) is the only way one can get a list of all the install components that have been defined. With the latest documentation, you'd never know this capability existed.