Add documentation section about what to expect from akka-http client

Lots of people come from using either JDK's UrlConnection or AsyncHttpClient. AFAIK both of them do not queue requests somewhere but either use a pooled connection, open up a new connection, or fail directly. It should be made clear that having a big (or even unlimited connection pool per host which AHC seems to set by default), isn't fair to a server, the connection, and might not even be the fastest way to run requests.

Our default settings and behavior differ with those libraries:

• default maximum of only 4 connections per host
• default maximum of only 32 ongoing requests (max-open-requests)
• way of backpressuring requests if the user code supports it via streaming APIs

We might consider increasing the defaults, though increasing the number of buffered, undispatched requests might not improve things.

This should probably tie in with #738.

Review the existing akka-http client documentation and issue #738 first. Add a section explaining the default four connections per host, 32 ongoing requests, request backpressure through streaming APIs, and how this differs from UrlConnection and AsyncHttpClient. Done means the documented defaults, behavior, and fairness considerations are clear to client users.