doc: experimental flag for global accessible APIs

Explicitly document that adding an API to the global scope requires
`semver-major` label. Waiving the `semver-major` requires a regular
TSC consensus process.

doc/contributing/collaborator-guide.md

@@ -437,6 +437,16 @@ For pull requests introducing new core modules:
 * Land with a [Stability Index][] of Experimental. The module must remain
   Experimental until a semver-major release.
 
+### Introducing new APIs on the global scope
+
+Exposing APIs to the global scope unconditionally without any CLI flags must
+always be labeled as `semver-major`. The `semver-major` label may be waived
+through the regular TSC consensus process.
+
+It is recommended to start with exposing APIs to the global scope with an
+experimental flag, without being labeled as `semver-major`. When the API is a
+release candidate, turn the flag on by default with a CLI flag to opt-out.
+
 ### Additions to Node-API
 
 Node-API provides an ABI-stable API guaranteed for future Node.js versions.