Add a section about the totality of functions

[amahboubi]

The main message of the discussion on the definition of subn seems to be to comment on the impact of the necessary totality of functions. This should be promoted as a section and include the stuff about odflt and the like which are now in containers.

[ybertot]

Agreed, I will look into it (already discussed with Enrico).

[ybertot]

This is included a pull request I just submitted.