Scan documentation is poor

The documentation for [`scanl`](https://hackage.haskell.org/package/text-1.2.5.0/docs/Data-Text.html#v:scanl) was probably taken straight from `Data.List` but is a little confusing in the context of `Text` (I also am not very clear on whether scans make much sense but that is above my pay grade):
```haskell
-- | /O(n)/ 'scanl' is similar to 'foldl', but returns a list of
-- successive reduced values from the left.
-- Performs replacement on invalid scalar values.
--
-- > scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
--
-- Note that
--
-- > last (scanl f z xs) == foldl f z xs.
```