Document MySQL and sub-second issue

See changelog at
https://github.com/rails/rails/blob/master/activerecord/CHANGELOG.md:
Don't run SQL if attribute value is not changed for update_attribute
method.

spec/models/gadget_spec.rb

@@ -15,6 +15,7 @@
     end
 
     it "should still generate a version when only the `updated_at` attribute is updated" do
+      # Plus 1 second because MySQL lacks sub-second resolution
       expect { gadget.update_attribute(:updated_at, Time.now + 1) }.to change{gadget.versions.size}.by(1)
     end
   end

spec/models/not_on_update_spec.rb

@@ -12,6 +12,7 @@
 
     it "increments the `:updated_at` timestamp" do
       before = record.updated_at
+      # Travel 1 second because MySQL lacks sub-second resolution
       Timecop.travel(1) do
         record.touch_with_version
       end

spec/models/widget_spec.rb

@@ -43,7 +43,8 @@
         subject { widget.versions.last.reify }
 
         it "should reset the value for the timestamp attrs for update so that value gets updated properly" do
-          Timecop.travel(5) do
+          # Travel 1 second because MySQL lacks sub-second resolution
+          Timecop.travel(1) do
             expect { subject.save! }.to change(subject, :updated_at)
           end
         end
@@ -255,6 +256,7 @@
 
         it "creates a version" do
           count = widget.versions.size
+          # Travel 1 second because MySQL lacks sub-second resolution
           Timecop.travel(1) do
             widget.touch_with_version
           end
@@ -263,6 +265,7 @@
 
         it "increments the `:updated_at` timestamp" do
           time_was = widget.updated_at
+          # Travel 1 second because MySQL lacks sub-second resolution
           Timecop.travel(1) do
             widget.touch_with_version
           end