Document `SemidirectProduct` for a matrix group and a vector space; or a group, a map to matrices (aka "representation") plus a vector space

[fingolfin]

This works but seems undocumented:

gap> F:=GF(3);; V:=F^3;; G:=SL(3,F);;
gap> H:=SemidirectProduct(G, V);
<matrix group of size 151632 with 3 generators>

Similar for this:

gap> G:=CyclicGroup(6);;M:=[[0,1],[1,1]]*Z(2);;
gap> rep:=GroupHomomorphismByImages(G,Group(M),[G.1],[M]);
gap> K:=SemidirectProduct(G,rep,GF(2)^2);
<pc group of size 24 with 4 generators>