Fix documentation of .:!=

I think this bit belongs in `.:?=`, by analogy with `.:!` and `.:?`.

Author: michaelpj

src/Data/Aeson/Types/FromJSON.hs

@@ -865,15 +865,15 @@ ifromJSON = iparse parseJSON
 (.:!) = explicitParseFieldMaybe' parseJSON
 
 -- | Retrieve the value associated with the given key of an 'Object'.
--- If the key is not present and the 'omittedField' is @'Just' x@ for some @x@,
+-- If the key is not present or the field is @null@ and the 'omittedField' is @'Just' x@ for some @x@,
 -- the result will be that @x@.
 --
 -- @since 2.2.0.0
 (.:?=) :: (FromJSON a) => Object -> Key -> Parser a
 (.:?=) = explicitParseFieldOmit omittedField parseJSON
 
 -- | Retrieve the value associated with the given key of an 'Object'.
--- If the key is not present or the field is @null@ and the 'omittedField' is @'Just' x@ for some @x@,
+-- If the key is not present and the 'omittedField' is @'Just' x@ for some @x@,
 -- the result will be that @x@.
 --
 -- This differs from '.:?=' by attempting to parse 'Null' the same as any