Improve documentation for CreateAssetAsync

[metatablecat]

Changes

The asset creation methods is currently only usable by plugins loaded in the LocalPlugin folder, this change marks that in the two relevant methods.

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[metatablecat]

Scrapped the TODO, ready for review.

[itsfrank]

Thank you for taking the time to improve the docs, I think this is definitely an improvement, I have some minor suggestions. I'm not extremely familiar with our docs language, so feel free to point out if my suggestions are inappropriate.