Improve usability of flow_graph to task_arena attachment

[isaevil]

This is a usability issue. The problem that attachment of flow_graph to specific task_arena looks confusing for users (see #419).
Consider simplifying oneTBB API in this sense. For example, one possible way is to accept task_arena instance in flow_graph constructor.
Also consider improving documentation for flow_graph.

[arunparkugan]

@isaevil is this issue still relevant?

[isaevil]

@arunparkugan yes.