docs: shorter explanation of shallow tracking

[Rich-Harris]

We devote way too much space to this nuance that doesn't really matter day-to-day. It's worth briefly explaining that values are tracked shallowly rather than deeply, but it's surprisingly hard to contrive a situation where this distinction actually matters. It doesn't warrant this much real estate.

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[dummdidumm]

I disagree - this has come up multiple times in Discord, I think it's fine to give it an extra example. With just the callout, I would have multiple questions when reading this as a user:

• what is user? Did I miss a previous example where user was used? -> this is why an extra code example is nice (also to explain the derived nuance)
• what does "shallowly" mean exactly here? (as a non-native speaker that word reads special)
• why does logging change the behavior? Why does JSON serialisation? -> either we explain why, or we just leave it out
• "oh so I can use inspect instead for my use case" -> click -> "oh this is dev time only" --> the previous sentence made it clear from the start that this is dev only