Note about removing portal-url, currentevents-url and helppage from default Sidebar

A note about the removal of portal-url, currentevents-url and helppage from
the default Sidebar as requested at the related bug report.
Follows up: Icb0e1b91e40bd682629f8a6f7291dbf8588b586

Bug: 53862
Change-Id: If7a624cd3a62c31af84a711f79ea396b53868fd2