Merge bitcoin/bitcoin#24226: rpc: warn that nodes ignore requests for old stale blocks

f39d9269eb rpc: warn that nodes ignore requests for old stale blocks (Sjors Provoost)

Pull request description:

  Adds warning to RPC help that `getblockfrompeer` is of little use for stale blocks that are more than a month old.

  This is an anti-fingerprinting measure. See `BlockRequestAllowed` in `net_processing`.

  It's been in Bitcoin Core since 2014, introduced in #2910 and later improved to not rely on checkpoints.
  Older and alternative clients might still serve these blocks, so not throwing an error.

  Allowing whitelisted nodes to fetch these blocks anyway might be nice.

ACKs for top commit:
  fjahr:
    Code review ACK f39d9269eb

Tree-SHA512: db88f9f7521289640c5e629c840dda1c2c3ab70d458e9e7136c60fbaeb02acfb36dc093502d83d4c098c331e22aab81bf8f4c4961d805e3bde0f8f3cfe68d968

src/net_processing.cpp

@@ -1743,6 +1743,8 @@ std::optional<std::string> PeerManagerImpl::FetchBlock(NodeId peer_id, const CBl
     LOCK(cs_main);
 
     // Mark block as in-flight unless it already is (for this peer).
+    // If the peer does not send us a block, vBlocksInFlight remains non-empty,
+    // causing us to timeout and disconnect.
     // If a block was already in-flight for a different peer, its BLOCKTXN
     // response will be dropped.
     if (!BlockRequested(peer_id, block_index)) return "Already requested from this peer";