Correct acts_as_event usage note and tighten followItem spec arg type

The header comment claimed search did not rely on acts_as_event; the
server-rendered search results page builds its work package links through
WorkPackage#event_url, so the note now reflects that search and atom feeds
both depend on it while the Activities subsystem uses its own providers.

Type the followItem spec helper from the method signature instead of
unknown, so the test states the argument contract explicitly.

frontend/src/app/core/global_search/input/global-search-input.component.spec.ts

@@ -37,7 +37,7 @@ describe('GlobalSearchInputComponent#followItem', () => {
   let searchInScopeArgs:string[];
   let context:Pick<GlobalSearchInputComponent, 'wpPath'|'selectedItem'> & { searchInScope:(scope:string) => void };
 
-  function callFollowItem(item:unknown):void {
+  function callFollowItem(item:Parameters<GlobalSearchInputComponent['followItem']>[0]):void {
     GlobalSearchInputComponent.prototype.followItem.call(context, item);
   }
 

lib_static/plugins/acts_as_event/lib/acts_as_event.rb

@@ -28,9 +28,9 @@
 # See COPYRIGHT and LICENSE files for more details.
 #++
 
-# This now only seems to be used when rendering atom responses.
+# The event_* methods defined here power the server-rendered search results page
-# Search as well as activities do not rely on it.
+# and atom feeds. The Activities subsystem renders through its own providers, so
-# Thus, whenever an atom link is removed for a resource, acts_as_event within that model can also be removed.
+# a model can drop acts_as_event only once neither search nor atom references it.
 
 module Redmine
   module Acts