Combines start and length of the document range. Used in Layout API.
Unlike the DocumentRange, the FixedRange cannot change after a document modification.
IN Layout API, it indicates a position of the RangedLayoutElement descendant in the document layout model.
The Range property of the layout element obtains the FixedRange object. Its FixedRange.Start and FixedRange.Length values correspond to the DocumentRange.Start and DocumentRange.Length values of the DocumentRange object obtained by the Range property of the object related to the layout element in the logical document model.