Do not use ford_fulkerson in max flow tests for large graphs.

The goal is to speed up a bit the maximum flow tests for large graphs.
The regular tests for maximum flow algorithms are enough for making sure
that ford_fulkerson is correct. Also reused residual network when possible.
Improve docstrings for flow functions.

Explain that these functions have to be explicitly imported
from the flow package.
Add docstrings to build_residual_network and add it to sphinix docs.
Add draft of 1.9 api changes for the flow package.

Based on the previous drafts posted at #1162 .