Improve documentation and add examples

in `Control.Lens.Traversal`
- typo in haddock for `deepOf`
- fix haddock for `elements`

in `Control.Lens.Fold`
- docs and example for `foldMapOf`
- docs and example for `foldOf`
Make it possible to force the TH generation of non-updateable field optics
Fixup doctest formatting
Merge pull request #509 from markus1189/docs

Improve documentation and add examples