Document GDScript "when" and clarify pattern guard docs

[tetrapod00]

Fixes #9786.

Adds an entry for "when" to the table, which links to "match".

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As for potential confusion about "pattern guards" vs "guard expressions":

• In the docs, both phrases only appear on this page.
• The engine itself has some errors that mention a "pattern guard" with an "expression". The phrase "guard expression" does not appear anywhere in engine error messages.
• The class reference never mentions "guard expressions" or "pattern guards".

So my impression is that you can create a match statement with a "pattern guard" which has an "expression". And you might call that expression a "guard expression", as the current docs section does. But these seem to be terms that are only implicitly defined in a one place in user-facing content, or perhaps terms from another language that are borrowed directly.

I changed the section header from "Pattern guards" to "Pattern guard expressions" so it can be found with ctrl+f both ways. Not too attached to that change.

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I mainly use C#, so a review from a GDScript expert would be appreciated.

[MajorGonzo]

As a new-ish Godot user, I agree that a single term should be used consistently in the documentation. Prevents confusion in us new-comers.

[mhilbrunner]

Thanks, merged. And thanks dalexeev for reviewing :)

[mhilbrunner]

Cherry-picked to 4.3 in #10038.