Update rules docs: pseudo elements, feature queries

[pepelsbey]

Description

I’ve been reading the docs and discovered a few places that could be improved. This PR is focused on basics/rules.mdx, but I have a few more improvements in other parts. Please let me know if I should use other PRs for them or combine everything in one place.

Here’s the following:

• ::before is a pseudo element, not pseudo class
• It’s preferable to use double colons to differentiate elements from classes (according to spec)
• It’s worth mentioning that pseudo classes/elements could be combined in a single string, not just nested
• @supports is a part of Feature queries, not Support queries

Checklist

Quality Assurance

• The code was formatted using Prettier (pnpm run format)
• The code has no linting errors (pnpm run lint)
• All tests are passing (pnpm run test)

Changes

• Tests have been added/updated
• Documentation has been added/updated

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[robinweser]

Nice improvements, thank you!