Third pass on doctest corrections.

This is the third scan over doctests in base. After this is merged, I think we (i.e. @Kleidukos) could setup a CI job to test them.

I'm using cabal-docspec to check doctest examples. I recommend reading through MANUAL.md (it's not long). It also contains base as an example.

While cabal-docspec is a very young tool, it's capable enough to run all doctests in kmettverse packages. (after a little of modifications, see manual for more).