info.xml - mark experimental tags as such

mark `<requires>` and `<ext>` as experimental, and link to the PR which
makes them functional.

docs/extensions/info-xml.md

@@ -141,6 +141,9 @@ Elements acceptable within `<compatibility>`
 
 ### `<ext>` {:#ext}
 
+!!! warning "Experimental"
+    This element does not (yet?) do anything. Functionality is pending the merge of PR [#9416](https://github.com/civicrm/civicrm-core/pull/9416).
+
 * Containing element: [`<requires>`](#requires)
 * Description: Specifies the unique name of one extension on which this extension is dependent.
 * Contains: text
@@ -275,6 +278,9 @@ Attributes acceptable for `<psr4>`
 
 ### `<requires>` {:#requires}
 
+!!! warning "Experimental"
+    This element does not (yet?) do anything. Functionality is pending the merge of PR [#9416](https://github.com/civicrm/civicrm-core/pull/9416).
+
 * Containing element: [`<extension>`](#extension)
 * Description: Used to to specify other extension on which this extension is dependent
 * Contains: elements