diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 4ebd634cd6..b791114ed9 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -114,7 +114,7 @@ numbers. New applications filter out passwords by adding the following `config.f
 defaults to `:debug` for all environments. The available log levels are: `:debug`,
 `:info`, `:warn`, `:error`, `:fatal`, and `:unknown`.
 
-* `config.log_tags` accepts a list of methods that the `request` object responds to. This makes it easy to tag log lines with debug information like subdomain and request id - both very helpful in debugging multi-user production applications.
+* `config.log_tags` accepts a list of: methods that the `request` object responds to, a `Proc` that accepts the `request` object, or something that responds to `to_s`. This makes it easy to tag log lines with debug information like subdomain and request id - both very helpful in debugging multi-user production applications.
 
 * `config.logger` accepts a logger conforming to the interface of Log4r or the default Ruby `Logger` class. Defaults to an instance of `ActiveSupport::Logger`.
 
diff --git a/railties/lib/rails/rack/logger.rb b/railties/lib/rails/rack/logger.rb
index 9962e6d943..5103c1e2ca 100644
--- a/railties/lib/rails/rack/logger.rb
+++ b/railties/lib/rails/rack/logger.rb
@@ -7,6 +7,9 @@ require 'rack/body_proxy'
 module Rails
   module Rack
     # Sets log tags, logs the request, calls the app, and flushes the logs.
+    #
+    # Log tags can be an Array containing: methods that the <tt>request</tt> object responds to, a Proc
+    # that accepts an instance of the <tt>request</tt> object, or something that responds to <tt>to_s</tt>.
     class Logger < ActiveSupport::LogSubscriber
       def initialize(app, taggers = nil)
         @app          = app