clarify timeout defaults

[wmorgan]

In https://linkerd.io/2/tasks/configuring-timeouts/, it currently says:

"If unspecified, the default timeout is 10 seconds."

This should be clarified to point out that this only applies if a service profile is set. Without a service profile set, there is no timeout on requests. (At least that's what @grampelberg tells me!)

We may also want to clarify whether the 10s default applies only to matched routes without a timeout config, or also to unmatched routes.