Fix and add missing haddock documentation.

``` unionR ``` and ``` unionL ``` seemed redundant so I removed them. The fact is my brain was numb after working on the documentation so I may have missed the purpose.

The complexity annotations also need fixing but I'm getting dizzy.
Also there are a few odd sounding formulation, but as I'm not a native English speaker,  I did not dare.

Edit : amended commit to include complexity annotation updates.