Explain advantages/disadvantages of different push transport modes

[AB-xdev]

https://vaadin.com/docs/latest/advanced/server-push#push.configuration.pushmode currently doesn't contain what the advantages/disadvantages of the different transport modes of push are.

There is also no further description in the JavaDocs.

It would be nice if there was a bit of explaining text or some sort of comparison highlighting advantages/disadvantages.

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