Merge bitcoin/bitcoin#30635: rpc: add optional blockhash to waitfornewblock, unhide wait methods in help

c6e2c31c55 rpc: unhide waitfor{block,newblock,blockheight} (Sjors Provoost)
0786b7509a rpc: add optional blockhash to waitfornewblock (Sjors Provoost)

Pull request description:

  The `waitfornewblock` is inherently racy as the tip may have changed since the last RPC call, and can even change during initial processing of this call.

  Add an optional `blockhash` argument so the caller can specify their current tip. Return immediately if our tip is different.

  I've made it fail if `LookupBlockIndex` fails. This should never happen if the user got the block hash from our RPC in the first place.

  Finally, the `waitfor{block,newblock,blockheight}` RPC methods are no longer hidden in `help`:
  - the changes in #30409 ensured these methods _could_ work in the GUI
  - #31785 removed the guards that prevented GUI users from using them
  - this PR makes `waitfornewblock` reliable

  So there's no more reason to hide them.

ACKs for top commit:
  TheCharlatan:
    Re-ACK c6e2c31c55
  ryanofsky:
    Code review ACK c6e2c31c55. Just rebased and tweaked documentation since last review.
  glozow:
    utACK c6e2c31c55

Tree-SHA512: 84a0c94cb9a2e4449e7a395cf3dce1650626bd852e30e0e238a1aafae19d57bf440bfac226fd4da44eaa8d1b2fa4a8c1177b6c716235ab862a72ff5bf8fc67ac

test/functional/rpc_blockchain.py

@@ -593,7 +593,8 @@ class BlockchainTest(BitcoinTestFramework):
         node.reconsiderblock(rollback_hash)
         # The chain has probably already been restored by the time reconsiderblock returns,
         # but poll anyway.
-        self.wait_until(lambda: node.waitfornewblock(timeout=100)['hash'] == current_hash)
+        self.wait_until(lambda: node.waitfornewblock(current_tip=rollback_header['previousblockhash'])['hash'] == current_hash)
+
         assert_raises_rpc_error(-1, "Negative timeout", node.waitfornewblock, -1)
 
     def _test_waitforblockheight(self):