[DOC] Add description of OptionParser#define_by_keywords

lib/optparse/kwargs.rb

@@ -7,12 +7,17 @@ class OptionParser
   #
   # :include: ../../doc/optparse/creates_option.rdoc
   #
-  def define_by_keywords(options, meth, **opts)
-    meth.parameters.each do |type, name|
+  # Defines options which set in to _options_ for keyword parameters
+  # of _method_.
+  #
+  # Parameters for each keywords are given as elements of _params_.
+  #
+  def define_by_keywords(options, method, **params)
+    method.parameters.each do |type, name|
       case type
       when :key, :keyreq
         op, cl = *(type == :key ? %w"[ ]" : ["", ""])
-        define("--#{name}=#{op}#{name.upcase}#{cl}", *opts[name]) do |o|
+        define("--#{name}=#{op}#{name.upcase}#{cl}", *params[name]) do |o|
           options[name] = o
         end
       end