Briefly document Vector<> variations

[esainane]

This continues #10329.

Packed*Array aliases seem universally preferred where available, so a link to the list of types seems appropriate.

LocalVector is used sparingly, so mentioning the intent and rough tradeoff involved seems right for an overview.

Bugsquad edit: closes #6259.

[esainane]

Please do check my understanding is correct - this is based off my reading of the source, and commentary from #10329, but there may be nuances which I'm missing.

[tetrapod00]

Approving only for style, and for not seeing anything obviously wrong from my casual understanding of the Godot codebase.

It's good to go as long as it is correct, IMO.

[esainane]

Cheers for all the reviews, and sorry about the delay! I should be able to take another look at this tonight.

[skyace65]

@esainane no rush, but are you planning to continue working on this?

[esainane]

Sorry for the delay; we live in rather interesting times. :|

I've fully rebased the PR, and tried to incorporate all of the feedback. Current HEAD is an attempted compromise by mentioning both LocalVector and Vector at the start before getting into details, which I think still flows naturally? It's getting a bit late here, so my apologies if there's something silly which slipped through.

[esainane]

I suspect this would read better if moved to the "Containers" section which immediately follows "Allocating memory", but wanted to provide an update for existing feedback first.

[AThousandShips]

LGTM

[Ivorforce]

Looks correct information-wise now.

[skyace65]

Grammar and spelling wise everything looks good

[skyace65]

Thanks!