message: doc: fix wording of alertnotify
  Since the removal of the PartitionChecker code in
  the documentation of alertnotify no longer matches the implementation.
  Instead simply document that alertnotify will be called when "an alert
  is raised".
diff: @@ -391,7 +391,7 @@ void SetupServerArgs(ArgsManager& argsman)
  argsman.AddArg("-version", "Print version and exit", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
  #if HAVE_SYSTEM
  - argsman.AddArg("-alertnotify=<cmd>", "Execute command when a relevant alert is received or we see a really long fork (%s in cmd is replaced by message)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
  + argsman.AddArg("-alertnotify=<cmd>", "Execute command when an alert is raised (%s in cmd is replaced by message)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
  #endif
  argsman.AddArg("-assumevalid=<hex>", strprintf("If this block is in the chain assume that it and its ancestors are valid and potentially skip their script verification (0 to verify all, default: %s, testnet: %s, signet: %s)", defaultChainParams->GetConsensus().defaultAssumeValid.GetHex(), testnetChainParams->GetConsensus().defaultAssumeValid.GetHex(), signetChainParams->GetConsensus().defaultAssumeValid.GetHex()), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
  argsman.AddArg("-blocksdir=<dir>", "Specify directory to hold blocks subdirectory for *.dat files (default: <datadir>)", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
  