Missing docstrings for a lot of G-set functionality

[fingolfin]

While working on PR #3309 I noticed that there are no docstrings for many functions for G-sets, e.g. is_regular or is_transitive (see content of that PR for a longer list to check).

We really should add these (often we can just copy & edit the existing docstrings for these functions which take a permutation group).

These should also appear in the manual under https://docs.oscar-system.org/stable/Groups/action/#G-Sets