Add documentation for Struct

mrblib/struct.rb

+##
+# Struct
 #
-#  Struct
-#
+# ISO 15.2.18
 class Struct
-  # 15.2.18.4.4
+
+  ##
+  # Calls the given block for each element of +self+
+  # and pass the respective element.
+  #
+  # ISO 15.2.18.4.4
   def each(&block)
     self.class.members.each{|field|
       block.call(self[field])
@@ -10,7 +16,12 @@ class Struct
     self
   end
 
-  # 15.2.18.4.5
+  ##
+  # Calls the given block for each element of +self+
+  # and pass the name and value of the respectiev
+  # element.
+  #
+  # ISO 15.2.18.4.5
   def each_pair(&block)
     self.class.members.each{|field|
       block.call(field.to_sym, self[field])
@@ -18,7 +29,12 @@ class Struct
     self
   end
 
-  # 15.2.18.4.7
+  ##
+  # Calls the given block for each element of +self+
+  # and returns an array with all elements of which
+  # block is not false.
+  #
+  # ISO 15.2.18.4.7
   def select(&block)
     ary = []
     self.class.members.each{|field|