Predefined tests for families should be properly documented

[JHahnHRO]

In GAP's method selection it is possible to define a necessary relationship between the families of the arguments, for example "The 1st argument of my method is a collection and the 2nd argument's family is the family of elements of said collection"
A number of such standard tests are actually predefined in GAP, e.g. the one above, but they do not seem to be documented anywhere except in the brains of certain experts (like @fingolfin who showed me these).

[fingolfin]

This is primarily about the family predicates in lib/fampred.g, although there are a few more.

I also noticed that those functions are all world writeable, too -- so perhaps the first thing to do would be to change their definitions to use BindGlobal (resp. BIND_GLOBAL if this file is loaded before the definition of BindGlobal).

Finally, I think that the documentation for what family predicates are doing is rather lacking -- in particular, it is missing examples. Grepping for family predicate doesn't reveal many hits. I think there should be an index entry for family predicate, pointing to paragraph somewhere (possibly in/after/near the InstallMethod documentation) explaining what a family predicate does, and also with at least one example.