Clarify advantage of intra-project parallelism

@pioterj commented on [Fri Oct 05 2018](https://github.com/gradle-guides/using-the-worker-api/issues/3)

We currently start the guide with:

> The Worker API provides the ability to break up the execution of a task action into discrete units of work and then to execute that work concurrently and asynchronously.

It imples what can be summarized with the feedback we received:

> The purpose of Worker API seems to be to parallelize a single task and make it leverage multiple threads, which is good for making a single task faster.

What worker API very importantly also provides is safe parallelism of tasks within a single project. Let's make sure we make this point clearly at the beginning of the guide.

Start with the opening Worker API guide text quoted in the issue and review how it describes concurrency. Update the introduction so it clearly includes safe parallelism among tasks within one project, then verify that the guide presents both benefits without implying that only a single task can be parallelized.

Written by the indexing model from the issue text.