Move "SpecialSearchResultsPrepend" note back to 1.21

Partial revert of I6a51e3ee: the hooks were actually added
before the branch point (in Ib3843314), not in I226349aa, which
merely added the release note.

The author of I226349aa has already backported the change.

Change-Id: Ifc2728133c6ca52ef866ce963c28bd3d06eab05a