Merging behaviour of extend() could be more explicit

[Altreus]

Although heavily implied by this sentence -

if a property of the first object is itself an object or array, it will be completely overwritten by a property with the same key in the second object

it is not made clear (in the sense of "I'll skim the docs to see how it behaves") that keys in later objects overwrite keys from earlier ones. Not a huge deal, and behaviour is as expected anyway, but I think it would be friendly to be more explicit about it.