document the direction of group isomorphisms (#3645)

for functions that return group isomorphisms

docs/src/Groups/grouphom.md

@@ -150,6 +150,15 @@ preimage(f::GAPGroupHomomorphism{S, T}, H::T) where S <: GAPGroup where T <: GAP
 
 ## Group isomorphisms
 
+For all functions that return group isomorphisms,
+we have the following rule about the direction of the result.
+
+If two groups are given as inputs then the domain of the returned isomorphism
+is the first given group and the codomain is the second.
+
+If one group is given then the domain of the result is this group,
+and the codomain is some new group constructed by the function.
+
 ```@docs
 is_isomorphic(G::GAPGroup, H::GAPGroup)
 is_isomorphic_with_map(G::GAPGroup, H::GAPGroup)