Document listen_for_error deprecation in API docs

Make sure it's documented in the API docs as deprecated as well. In case
we do another release before 4.0, this can be included.

[skip changeset]

lib/appsignal/helpers/instrumentation.rb

@@ -300,7 +300,10 @@ def monitor_single_transaction(name, env = {}, &block)
       # @see .send_error
       # @see https://docs.appsignal.com/ruby/instrumentation/integrating-appsignal.html
       #   AppSignal integration guide
+      # @see https://docs.appsignal.com/ruby/instrumentation/exception-handling.html
+      #   Exception handling guide
       #
+      # @deprecated Use `rescue => error` with {.report_error} instead.
       # @param tags [Hash, nil]
       # @param namespace [String] the namespace for this error.
       # @yield yields the given block.