document nodejs_process_v2 compat flag

[guybedford]

Summary

This adds documentation for the new enable_nodejs_process_v2 compat flag which will be auto-enabled on September 15.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• 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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.

[irvinebroque]

• Yagiz comments but stamp so you have it

[vicb]

Thanks!