Added comment on required invocation order

To use vtkUnstructuredGrid's InsertNextCell, you must have called its
Allocate method.  This was documented on Allocate, but not on
InsertNextCell.  Someone with a segfault in InsertNextCell will not
be looking at the documentation for Allocate.  This change adds a line
to the documentation of each overload of InsertNextCell to inform that
Allocate must be called first.

Change-Id: Idacf2081ded84a27fc574e85904e9082ff5c8536
Merge topic 'fix-docs-on-call-order-dependency' into master

a8d27882 Added comment on required invocation order