[Doc] Improve documentation of PP

* Remove mention to `require 'pp'` for `pretty_inspect`
* Mention the need to add `require 'pp'` to customize
  `#pretty_print(pp)` method

lib/pp.rb

@@ -46,6 +46,7 @@ require 'prettyprint'
 #
 # To define a customized pretty printing function for your classes,
 # redefine method <code>#pretty_print(pp)</code> in the class.
+# Note that <code>require 'pp'</code> is needed before redefining <code>#pretty_print(pp)</code>.
 #
 # <code>#pretty_print</code> takes the +pp+ argument, which is an instance of the PP class.
 # The method uses #text, #breakable, #nest, #group and #pp to print the
@@ -632,10 +633,6 @@ end
 module Kernel
   # Returns a pretty printed object as a string.
   #
-  # In order to use this method you must first require the PP module:
-  #
-  #   require 'pp'
-  #
   # See the PP module for more information.
   def pretty_inspect
     PP.pp(self, ''.dup)