Incomplete documentation for distributed property maps

[The documentation](https://github.com/boostorg/graph_parallel/blob/master/doc/distributed_property_map.rst) for distributed property maps is missing any mention of the `GlobalMap` parameter of distributed property maps.

There is a complete example of how to use distributed property maps [here](https://github.com/boostorg/graph_parallel/blob/master/test/distributed_property_map_test.cpp).

To fix this issue, the documentation would need to be brought up to date with the actual use of distributed maps.