`private def` breaks RDoc. Move meathod to preserve the docs.

The rdoc parser seems to trip on the `private def` construct.
Public methods following a method defined with `private def` are not
visible inside the module docs but are appended to the top-most module.

For example the method `ActiveRecord::QueryMethods#distinct` was listed
under `ActiveRecord#distinct`.

/cc @sgrif

activerecord/lib/active_record/relation/query_methods.rb

@@ -604,12 +604,6 @@ def or!(other) # :nodoc:
       self
     end
 
-    private def structurally_compatible_for_or?(other) # :nodoc:
-      Relation::SINGLE_VALUE_METHODS.all? { |m| send("#{m}_value") == other.send("#{m}_value") } &&
-        (Relation::MULTI_VALUE_METHODS - [:extending]).all? { |m| send("#{m}_values") == other.send("#{m}_values") } &&
-        (Relation::CLAUSE_METHODS - [:having, :where]).all? { |m| send("#{m}_clause") != other.send("#{m}_clause") }
-    end
-
     # Allows to specify a HAVING clause. Note that you can't use HAVING
     # without also specifying a GROUP clause.
     #
@@ -1095,6 +1089,12 @@ def check_if_method_has_arguments!(method_name, args)
       end
     end
 
+    def structurally_compatible_for_or?(other)
+      Relation::SINGLE_VALUE_METHODS.all? { |m| send("#{m}_value") == other.send("#{m}_value") } &&
+        (Relation::MULTI_VALUE_METHODS - [:extending]).all? { |m| send("#{m}_values") == other.send("#{m}_values") } &&
+        (Relation::CLAUSE_METHODS - [:having, :where]).all? { |m| send("#{m}_clause") != other.send("#{m}_clause") }
+    end
+
     def new_where_clause
       Relation::WhereClause.empty
     end