Document rules to use GIT_REFLOG_ACTION variable in the scripted
Porcelain.  git-rebase--interactive locally violates them, but it
is a leaf user that does not call out to or dot-source other
scripts, so it does not urgently need to be fixed.

* jc/reflog-doc:
  setup_reflog_action: document the rules for using GIT_REFLOG_ACTION