lib/repo-pull: Fix docs for n-network-retries option
lib/repo-pull: Fix docs for n-network-retries option

Add "Since: 2018.6" and copy the docs to
ostree_repo_pull_from_remotes_async() which also supports it.

Closes: #1812
Approved by: jlebon

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