Add documentation concerning interval test

[edouardmenayde]

Summary of changes

I added documentation for interval testing because I was searching for the feature in the doc found nothing and finally found it in the code thus I want to improve the documentation.

I could not document directly the implementation of Enumerable and found no way of doing it.

[coveralls]

Coverage increased (+0.07%) to 68.072% when pulling abe0378 on edouardmenayde:doc/interval into 0a736dc on bitwalker:master.

[coveralls]

Coverage increased (+0.07%) to 68.072% when pulling abe0378 on edouardmenayde:doc/interval into 0a736dc on bitwalker:master.

[bitwalker]

Thanks!