Merge branch 'jl/submodule-mv-checkout-caveat'

Merge branch 'jl/submodule-mv-checkout-caveat'

With a submodule that was initialized in an old fashioned way
without gitlinks, switching branches in the superproject between
the one with and without the submodule may leave the submodule
working tree with its embedded repository behind, as there may be
unexpendable state there. Document and warn users about this.

* jl/submodule-mv-checkout-caveat:
  rm: better document side effects when removing a submodule
  mv: better document side effects when moving a submodule