Documentation: Clearly state that OkHttpClient should be a singleton

[jfontsaballs]

I learned from StackOverflow that a single instance of OkHttpClient should be created and that it should be shared among all services in the application that need to communicate via HTTP. I haven't been able to find anything definitive in the docs on how the OkHttpClient is designed to be used.

I'd like to request that this information be included in the overview page of the docs https://square.github.io/okhttp/ in order to have a trustworthy and easy to find source for that piece of information. All examples in the docs just create a new instance of the client.

[yschimke]

Yep, it should probably go on the main doc page.

It's here currently https://square.github.io/okhttp/5.x/okhttp/okhttp3/-ok-http-client/index.html