docs&refactor(useWebSocket/useEventSource)!: uniformize immediate behaviour

[ferferga]

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Description

As I was pointed here, there's a mismatch between the behaviour of the immediate setting in both useWebSocket and useEventSource: in all the others composables, the immediate flag just specifies that the composable should "act" as soon as it's invoked, but in useWebSocket and useEventSource it also opts-out all the reactivity from the refs.

In my opinion, opting out from the reactivity doesn't make sense, since that's essentially the goal of VueUse (provide reactive wrappers around common browser APIs).

I believe the intended behaviour is to work like all the other composables (1) (2) (that PR was correct according to docs, but incorrect for my "concept" of immediate).

Feel free to close this PR in case the current behaviour is correct.

Additional context

Not sure either if this should be considered a breaking change.