Add note about running next without a sub-command

[refactorized]

This should be made explicitly clear because other code may just call next instead of next dev (e.g. https://github.com/netlify/framework-info/blob/main/src/frameworks/next.json).

Being in the ninth circle of debug hell right now I could not take it for granted that the two were the same till I saw some kind of confirmation, either from the docs or from the source. Source is great, but calling it out in the docs would saved me some time.

Bug

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Documentation / Examples

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