docs: Make DIRECT_URL naming consistent with the previous pattern

[kof]

Describe this PR

Current naming makes a mess in any env, because DIRECT_URL doesn't tell what that is, at the very least it has to stay consistent with the previous pattern, in ideal case both will be renamed because they are semantically wrong.

Changes

More consistent env variable naming

What issue does this fix?

Now people are copying bad naming and getting "DIRECT_URL" as a global env variable which says nothing about its usage and isn't consistent with Prisma's other url

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[jharrell]

Hi there! Thank you very much for the contribution 🙏

Unfortunately, I don't think we'll be able to include this. While you're correct, DATABASE_DIRECT_URL is a more clear variable, we have several years of content pointing to the current setup of DATABASE_URL and DIRECT_URL.

Again, thank you very much, and I'll also bring notes back to our team to consider this naming change in the future.