Add a description of #defined statements

[daveraja]

For some reason I always forget what keyword to use when I want to disable the warning messages about predicates with no head definitions. I end up searching through old code or the mailing list to find a reference. So I think it would be great to have it documented in the guide.

[MaxOstrowski]

As I just had the same problem, as an intermediate solution for everyone who finds this, it is:

#defined foo/2.