Document how connection pooling works in reqwest

[ranger-ross]

Hey there, thanks for a great library.

Coming from the Java ecosystem (OkHTTP) the connection pooling options in reqwest feel quiet limited.
I can see various feature requests (#2445, #2424, etc) to added more customizablity to the pooling behavior.

I think it would be a good idea to document how the connection pooling behavior works (probably on the Client struct) so that library users can be more informed when setting pool configurations.