rpc: unhide waitfor{block,newblock,blockheight}

They are now reliable. An earlier commit dropped their
IsRPCRunning() guards so they also work in the GUI.

doc/release-30635.md

@@ -1,4 +1,5 @@
 Updated RPCs
 ------------
 
-- The waitfornewblock RPC takes an optional `current_tip` argument. (#30635)
+- The waitfornewblock now takes an optional `current_tip` argument. It is also no longer hidden. (#30635)
+- The waitforblock and waitforblockheight RPCs are no longer hidden.  (#30635)