Improve documentation of UnknownArrayHandle::IsType

Under some circumstances, `IsType` can return false for a type that
exactly matches the type used to create the `UnknownArrayHandle`.
This is because types like `ArrayHandleCast` get converted to
the array they contain. Thus, `IsType` will return false when it
is expected to be true. In most cases, you should use
`CanConvert`. The `IsType` documentation now points you there.
5 jobs for unknownarrayhandle-istype-documentation in 0 seconds
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passed cdash-buildbot

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

00:09:32

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

00:15:40

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

00:21:47

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

00:10:50