doc: add experimental modules lifetime policy

doc/api/documentation.md

@@ -47,6 +47,14 @@ The stability indexes are as follows:
 >
 > Experimental features leave the experimental status typically either by
 > graduating to stable, or are removed without a deprecation cycle.
+>
+> Collaborators who introduce experimental modules are expected to take
+> ownership and drive each experiment in a timely manner to a clear outcome:
+> either promotion to stable or removal.
+>
+> If an experimental feature has reached mainstream adoption such that breaking
+> changes are not realistically possible without ecosystem breakage, it should
+> be considered stable and either be promoted or be removed after a deprecation cycle.
 
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doc/contributing/collaborator-guide.md

@@ -462,6 +462,11 @@ For pull requests introducing new core modules:
 * Land only after sign-off from at least two TSC voting members.
 * Land with a [Stability Index][] of Experimental. The module must remain
   Experimental until a semver-major release.
+* Introducing an Experimental feature means taking ownership of the experiment
+  and committing to a clear outcome: either promoting it to stable or removing
+  it in a timely manner. Because experimental features can be vulnerable to
+  security issues, the author is also expected to help assess and patch
+  vulnerabilities.
 
 ### Introducing new APIs on the global scope