clarify timeout defaults

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.

Start with the timeout documentation at https://linkerd.io/2/tasks/configuring-timeouts/. Verify when the 10-second default applies, including whether a service profile is required and how matched versus unmatched routes behave. Update the wording to clarify those cases and confirm the page accurately describes the resulting request timeout behavior.

Written by the indexing model from the issue text.