docs: update environment.md - mention the vitest's jsdom includes fetch

[dwjohnston]

Description

I spent some time wondering why fetch works using vitest, where some additional manual steps are required for setup with jest. It's helpful to have that mentioned in the docs.

See discussion here: #6428

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[sheremet-va]

Can you explain how this is helpful? Do we need to monitor Jest now to update this statement if they also inject fetch? I don't think comparisons to Jest are useful outside of a migration guide.

[dwjohnston]

@sheremet-va the change to the docs doesn't make any mention of Jest. I've mentioned it in the PR just as motivation/context for my raising the PR.

What it highlights is that vitest doesn't' use stock standard jsdom - there is some amount of modifications that vitest is applying to jsdom.