Missing docstrings for `permutation_group`, `pc_group`, `fp_group`

[fingolfin]

Some time ago @joschmitt (IIRC) improved things by adding methods for permutation_group, pc_group, fp_group that take a group and return an isomorphic group but which is a permutation/pc/fp group -- before we did (and still do) the same with the types, i.e. PermGroup(G) does the same as permutation_group(G).

However, these seem to be no docstrings for either of these conversions (i.e. either using the snake_case function or the CamelCase type names).

These should be added, and also reference isomorphism ("if you also need the isomorphism, please use isomorphism" or so)

[joschmitt]

This is documented:

Oscar.jl/src/Groups/homomorphisms.jl

Line 746 in 5b325bd

FPGroup(G::T) where T <: Union{GAPGroup, GrpAbFinGen}

There is just no chance to find it because the doc string is attached to a very weird signature as far as I can see.

[lgoettgens]

I would suggest to wait with this a bit until I had another chance to try to resolve #4616, as that may then change how these functions are split over Oscar and AA, and where to put their docstrings.