Merge branch 'jt/clone-recursesub-ref-advise'

The interaction between "git clone --recurse-submodules" and alternate object store was ill-designed. The documentation and code have been taught to make more clear recommendations when the users see failures. * jt/clone-recursesub-ref-advise: submodule--helper: advise on fatal alternate error Doc: explain submodule.alternateErrorStrategy