Make observer enable/disable documentation more informative.

* Moved the simplest case--enable/disable all on all--to the top.
* Made clear what "ORM" means to avoid having to teach people how to solve "uninitialized constant ORM"
  errors in their test reports.

activemodel/lib/active_model/observer_array.rb

@@ -17,6 +17,11 @@ def disabled_for?(observer)
 
     # Disables one or more observers. This supports multiple forms:
     #
+    #   ORM.observers.disable :all
+    #     # => disables all observers for all models subclassed from 
+    #     #    an ORM base class that includes ActiveModel::Observing
+    #     #    e.g. ActiveRecord::Base
+    #
     #   ORM.observers.disable :user_observer
     #     # => disables the UserObserver
     #
@@ -27,9 +32,6 @@ def disabled_for?(observer)
     #   ORM.observers.disable :observer_1, :observer_2
     #     # => disables Observer1 and Observer2 for all models.
     #
-    #   ORM.observers.disable :all
-    #     # => disables all observers for all models.
-    #
     #   User.observers.disable :all do
     #     # all user observers are disabled for
     #     # just the duration of the block
@@ -40,6 +42,11 @@ def disable(*observers, &block)
 
     # Enables one or more observers. This supports multiple forms:
     #
+    #   ORM.observers.enable :all
+    #     # => enables all observers for all models subclassed from 
+    #     #    an ORM base class that includes ActiveModel::Observing
+    #     #    e.g. ActiveRecord::Base
+    #
     #   ORM.observers.enable :user_observer
     #     # => enables the UserObserver
     #
@@ -51,9 +58,6 @@ def disable(*observers, &block)
     #   ORM.observers.enable :observer_1, :observer_2
     #     # => enables Observer1 and Observer2 for all models.
     #
-    #   ORM.observers.enable :all
-    #     # => enables all observers for all models.
-    #
     #   User.observers.enable :all do
     #     # all user observers are enabled for
     #     # just the duration of the block