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/api/documentation.md

@@ -66,6 +66,16 @@ used. Bugs or behavior changes may surprise users when Experimental API
 modifications occur. To avoid surprises, use of an Experimental feature may need
 a command-line flag. Experimental features may also emit a [warning][].
 
+### 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.
+
 ## Stability overview
 
 <!-- STABILITY_OVERVIEW_SLOT_BEGIN -->