Merge #19958: doc: Better document features of feelers

2ea62ca Improve docs about feeler connections (Gleb Naumenko)

Pull request description:

  "feeler" and "test-before-evict" are two different strategies suggest in [Eclipse Attacks on Bitcoin’s Peer-to-Peer Network](https://www.usenix.org/system/files/conference/usenixsecurity15/sec15-paper-heilman.pdf). In our codebase, we use `ConnType::FEELER` to implement both.

  It is confusing, up to the point that our documentation was just incorrect.

  This PR:
  - ~clarifies this aspect by renaming "ConnType::FEELER" to "ConnType::PROBE", meaning that this connections only probes that the node is operational, and then disconnects.~
  - fixes the documentation

ACKs for top commit:
  amitiuttarwar:
    ACK 2ea62ca. thank you!
  practicalswift:
    ACK 2ea62ca

Tree-SHA512: c9c03c09eefeacec28ea199cc3f697b0a98723f2f849f7a8115edc43791f8165e296e0e25a82f0b5a4a781a7de38c8954b48bf74c714eba02cdc21f7460673e5

src/net.cpp

@@ -1811,7 +1811,7 @@ void CConnman::SetTryNewOutboundPeer(bool flag)
 
 // Return the number of peers we have over our outbound connection limit
 // Exclude peers that are marked for disconnect, or are going to be
-// disconnected soon (eg one-shots and feelers)
+// disconnected soon (eg ADDR_FETCH and FEELER)
 // Also exclude peers that haven't finished initial connection handshake yet
 // (so that we don't decide we're over our desired connection limit, and then
 // evict some peer that has finished the handshake)

src/net.h

@@ -153,10 +153,19 @@ enum class ConnectionType {
     MANUAL,
 
     /**
-     * Feeler connections are short lived connections used to increase the
-     * number of connectable addresses in our AddrMan. Approximately every
-     * FEELER_INTERVAL, we attempt to connect to a random address from the new
-     * table. If successful, we add it to the tried table.
+     * Feeler connections are short-lived connections made to check that a node
+     * is alive. They can be useful for:
+     * - test-before-evict: if one of the peers is considered for eviction from
+     *   our AddrMan because another peer is mapped to the same slot in the tried table,
+     *   evict only if this longer-known peer is offline.
+     * - move node addresses from New to Tried table, so that we have more
+     *   connectable addresses in our AddrMan.
+     * Note that in the literature ("Eclipse Attacks on Bitcoin’s Peer-to-Peer Network")
+     * only the latter feature is referred to as "feeler connections",
+     * although in our codebase feeler connections encompass test-before-evict as well.
+     * We make these connections approximately every FEELER_INTERVAL:
+     * first we resolve previously found collisions if they exist (test-before-evict),
+     * otherwise connect to a node from the new table.
      */
     FEELER,