remove caution comment

I think this was inherited from flagFile, but the reasons to use caution
when using flagFile (that it makes code to satisfy a property only run once)
don't apply when using onChangeFlagOnFail.

src/Propellor/Property.hs

@@ -61,8 +61,6 @@ onChange = combineWith $ \p hook -> do
 -- With `onChange`, if y fails, the property x `onChange` y returns
 -- `FailedChange`. But if this property is applied again, it returns
 -- `NoChange`. This behavior can cause trouble...
---
--- Use with caution.
 onChangeFlagOnFail
 	:: (Combines (Property x) (Property y))
 	=> FilePath