lib/repo: Clean up OstreeRepo docs
lib/repo: Clean up OstreeRepo docs

This fixes typos and grammar in the docs for OstreeRepo, and copies the
information about OSTREE_REPO_MODE_BARE_USER_ONLY from ostree-core.h

Closes: #1725
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree