Merge branch 'wk/submodule-on-branch'

Merge branch 'wk/submodule-on-branch'

Make sure 'submodule update' modes that do not detach HEADs can
be used more pleasantly by checking out a concrete branch when
cloning them to prime the well.

* wk/submodule-on-branch:
  Documentation: describe 'submodule update --remote' use case
  submodule: explicit local branch creation in module_clone
  submodule: document module_clone arguments in comments
  submodule: make 'checkout' update_module mode more explicit