Document the process.env handling and compat flags

[jasnell]

Refs: cloudflare/workerd#3311

Summary

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Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[vicb]

I added inline comments.

It would be great to add details to https://developers.cloudflare.com/workers/configuration/secrets/ -> mention the flag and process.env

Thanks

[vicb]

What would be awesome is to document disallow_importable_env as well.

That is:

• Create a file in src/content/compatibility-flags/
• Link the flag where it is referenced in the content.

Thanks for documenting this 🎉

[jasnell]

I will add disallow_importable_env in a separate PR (will add it here: #24083)