added docs to String#at

activesupport/lib/active_support/core_ext/string/access.rb

@@ -1,6 +1,31 @@
 require 'active_support/multibyte'
 
 class String
+  # If you pass a single Fixnum, returns a substring of one character at that
+  # position. The first character of the string is at position 0, the next at
+  # position 1, and so on. If a range is supplied, a substring containing
+  # characters at offsets given by the range is returned. In both cases, if an
+  # offset is negative, it is counted from the end of the string. Returns nil
+  # if the initial offset falls outside the string. Returns an empty string if
+  # the beginning of the range is greater than the end of the string.
+  #
+  #   str = "hello"
+  #   str.at(0)      #=> "h"
+  #   str.at(1..3)   #=> "ell"
+  #   str.at(-2)     #=> "l"
+  #   str.at(-2..-1) #=> "lo"
+  #   str.at(5)      #=> nil
+  #   str.at(5..-1)  #=> ""
+  #
+  # If a Regexp is given, the matching portion of the string is returned.
+  # If a String is given, that given string is returned if it occurs in
+  # the string. In both cases, nil is returned if there is no match.
+  #
+  #   str = "hello"
+  #   str.at(/lo/) #=> "lo"
+  #   str.at(/ol/) #=> nil
+  #   str.at("lo") #=> "lo"
+  #   str.at("ol") #=> nil
   def at(position)
     self[position]
   end