[Doc] Describe the stance of WEBrick about its security and utilization

WEBrick is not recommended for the production use.  We need to explicitly
describe this fact in the document to avoid troubles due to misunderstanding.

lib/webrick.rb

@@ -15,6 +15,11 @@
 # WEBrick also includes tools for daemonizing a process and starting a process
 # at a higher privilege level and dropping permissions.
 #
+# == Security
+#
+# *Warning:* WEBrick is not recommended for production.  It only implements
+# basic security checks.
+#
 # == Starting an HTTP server
 #
 # To create a new WEBrick::HTTPServer that will listen to connections on port
@@ -139,9 +144,9 @@
 # servers.  See WEBrick::HTTPAuth, WEBrick::HTTPAuth::BasicAuth and
 # WEBrick::HTTPAuth::DigestAuth.
 #
-# == WEBrick as a Production Web Server
+# == WEBrick as a daemonized Web Server
 #
-# WEBrick can be run as a production server for small loads.
+# WEBrick can be run as a daemonized server for small loads.
 #
 # === Daemonizing
 #