Add information on what backends support RequestOptions.readTimeout to the scaladoc of RequestOptions

[Awethon]

It's quite hard to figure out whether this option will apply or be ignored. Having this information available as a scaladoc would be very handy.

[adamw]

Can you maybe create a PR?

[Awethon]

@adamw I wish I knew for which of the client backends it was implemented. Will take a while to fully cover all the supported backends.