Describe ActiveSupport core extensions for BigDecimal, JSON support, …

…and instance variable names.

guides/source/active_support_core_extensions.md

@@ -418,6 +418,14 @@ TIP: Since `with_options` forwards calls to its receiver they can be nested. Eac
 
 NOTE: Defined in `active_support/core_ext/object/with_options.rb`.
 
+### JSON support
+
+Active Support provides a better implemention of `to_json` than the json gem ordinarily provides for Ruby objects.  This is because some classes, like Hash and OrderedHash, needs special handling in order to provide a proper JSON representation.
+
+Active Support also provides an implementation of `as_json` for the Process::Status class.
+
+NOTE: Defined in `active_support/core_ext/object/to_json.rb`.
+
 ### Instance Variables
 
 Active Support provides several methods to ease access to instance variables.
@@ -439,6 +447,39 @@ C.new(0, 1).instance_values # => {"x" => 0, "y" => 1}
 
 NOTE: Defined in `active_support/core_ext/object/instance_variables.rb`.
 
+#### `instance_values`
+
+The method `instance_values` returns a hash that maps instance variable names without "@" to their
+corresponding values. Keys are strings:
+
+```ruby
+class C
+  def initialize(x, y)
+    @x, @y = x, y
+  end
+end
+
+C.new(0, 1).instance_values # => {"x" => 0, "y" => 1}
+```
+
+NOTE: Defined in `active_support/core_ext/object/instance_variables.rb`.
+
+#### `instance_variable_names`
+
+The method `instance_variable_names` returns an array.  Each name includes the "@" sign.
+
+```ruby
+class C
+  def initialize(x, y)
+    @x, @y = x, y
+  end
+end
+
+C.new(0, 1).instance_variable_names # => ["@x", "@y"]
+```
+
+NOTE: Defined in `active_support/core_ext/object/instance_variables.rb`.
+
 ### Silencing Warnings, Streams, and Exceptions
 
 The methods `silence_warnings` and `enable_warnings` change the value of `$VERBOSE` accordingly for the duration of their block, and reset it afterwards:
@@ -2011,8 +2052,31 @@ NOTE: Defined in `active_support/core_ext/integer/inflections.rb`.
 
 Extensions to `BigDecimal`
 --------------------------
+### `to_s`
+The method `to_s` is aliased to `to_formatted_s`. This provides a convenient way to display a BigDecimal value in floating-point notation:
 
-...
+```ruby
+BigDecimal.new(5.00, 6).to_s  # => "5.0"
+```
+
+### `to_formatted_s`
+Te method `to_formatted_s` provides a default specifier of "F".  This means that a simple call to `to_formatted_s` or `to_s` will result in floating point representation instead of engineering notation:
+
+```ruby
+BigDecimal.new(5.00, 6).to_formatted_s  # => "5.0"
+```
+
+and that symbol specifiers are also supported:
+
+```ruby
+BigDecimal.new(5.00, 6).to_formatted_s(:db)  # => "5.0"
+```
+
+Engineering notation is still supported:
+
+```ruby
+BigDecimal.new(5.00, 6).to_formatted_s("e")  # => "0.5E1"
+```
 
 Extensions to `Enumerable`
 --------------------------