Highlight that config.linkerd.io/skip-outbound-ports overrides defaults

[muenchdo]

Today we got bitten by the fact that setting config.linkerd.io/skip-outbound-ports overrides the default list of skipped ports. While the proxy configuration docs do mention it (see below), I think it would be nice to specifically highlight this caveat. This might include referencing the default list of skipped outbound ports, so it can be reflected in a custom configuration.

This is useful for **overriding** the default configurations of auto-injected proxies.

[adleong]

@muenchdo this is a good point. Are you interested in submitting a PR to the website to help clarify this?