Clarify ScanExtended

Make it more clear that ScanExtended increases the array size by
1, what ranges the exclusive and inclusive scan cover, and the
features of the first and last entries.
16 jobs for scan-extended in 0 seconds (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

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

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

passed cdash-buildbot

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

00:12:25

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

00:12:25

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

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

00:18:48

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

00:18:45

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

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

00:25:01

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

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

00:33:06

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

00:33:31