Documentation style

[nward]

As mentioned in a comment on #199, I think we should have a conscious decision on what we are documenting in params etc. documentations.

Should we be documenting:
a) The behaviour that FreeRADIUS will have if the user sets a parameter, as we currently do, or;
b) The behaviour that this module will have - i.e. that x parameter in y freeradius module will be set to value z

My proposal is that documentation should target (b), as FreeRADIUS' behaviour can change, and I don't think we can capture that in a reasonable way while supporting multiple versions.