Move to_time to DateTime compatibility.rb file

We are overriding it in `Time` and `ActiveSupport::TimeWithZone` so
there's no point in having it in the `DateAndTime::Compatibility`
module. Also add some docs for the `to_time` implementations.

activesupport/lib/active_support/core_ext/date_and_time/compatibility.rb

@@ -11,9 +11,5 @@ module Compatibility
     # this behavior, but new apps will have an initializer that sets
     # this to true, because the new behavior is preferred.
     mattr_accessor(:preserve_timezone, instance_writer: false) { false }
-
-    def to_time
-      preserve_timezone ? getlocal(utc_offset) : getlocal
-    end
   end
 end

activesupport/lib/active_support/core_ext/date_time/compatibility.rb

@@ -2,4 +2,14 @@
 
 class DateTime
   include DateAndTime::Compatibility
+
+  remove_possible_method :to_time
+
+  # Either return an instance of `Time` with the same UTC offset
+  # as +self+ or an instance of `Time` representing the same time
+  # in the the local system timezone depending on the setting of
+  # on the setting of +ActiveSupport.to_time_preserves_timezone+.
+  def to_time
+    preserve_timezone ? getlocal(utc_offset) : getlocal
+  end
 end

activesupport/lib/active_support/core_ext/time/compatibility.rb

@@ -6,6 +6,8 @@ class Time
 
   remove_possible_method :to_time
 
+  # Either return +self+ or the time in the local system timezone depending
+  # on the setting of +ActiveSupport.to_time_preserves_timezone+.
   def to_time
     preserve_timezone ? self : getlocal
   end

activesupport/lib/active_support/time_with_zone.rb

@@ -326,7 +326,9 @@ def to_datetime
       @to_datetime ||= utc.to_datetime.new_offset(Rational(utc_offset, 86_400))
     end
 
-    # Returns an instance of <tt>Time</tt>
+    # Returns an instance of +Time+, either with the same UTC offset
+    # as +self+ or in the local system timezone depending on the setting
+    # of +ActiveSupport.to_time_preserves_timezone+.
     def to_time
       if preserve_timezone
         @to_time_with_instance_offset ||= getlocal(utc_offset)