Document grace and keep, and how to deal with problematic servers
Document grace and keep, and how to deal with problematic servers

This commit comes with the realization that finding a workaround
for #1799 is even harder than I thought.

Unfortunately we cannot close issue #1799 for the reasons described
in the documentation.

Thanks to @nigoroll for valuable feedback.

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache