Mention the 24-hour restriction on unpublish (#15833)

* Mention the 24-hour restriction on unpublish

As detailed in http://blog.npmjs.org/post/141905368000/changes-to-npms-unpublish-policy

* Clarify that 24 hr unpublish policy only applies to default registry

PR-URL: #15833
Credit: @carols10cents 
Reviewed-By: @zkat

Author: carols10cents

doc/cli/npm-unpublish.md

@@ -27,6 +27,11 @@ Even if a package version is unpublished, that specific name and
 version combination can never be reused.  In order to publish the
 package again, a new version number must be used.
 
+With the default registry (`registry.npmjs.org`), unpublish is
+only allowed with versions published in the last 24 hours. If you
+are trying to unpublish a version published longer ago than that,
+contact support@npmjs.com.
+
 The scope is optional and follows the usual rules for `npm-scope(7)`.
 
 ## SEE ALSO