Add clarification for DEVICE_SOURCES

The `vtkm_add_target_information` CMake function has a `DEVICE_SOURCES`
option that allows you to list which sources need to be compiled with a
device compiler. Added some hints to the documentation about which
source code needs to be listed in `DEVICE_SOURCES`.
5 jobs for device-sources-clarification (queued for 1 second)
Status Name Job ID Coverage
  External
passed cdash-buildbot

passed vtk-m-dragnipur-osx-shared-debug+32bit_ids+64bit_coords+clang+tbb

00:01:49

passed vtk-m-dragnipur-osx-static-release+64bit_coords+clang+tbb

00:00:31

running vtk-m-osheim-windows-shared-debug+ninja-vs-2017+tbb

1247:36:08

passed vtk-m-osheim-windows-static-release+ninja-vs-2017+tbb

00:01:58