Document 1 second TTL on (synthetic) error objects
Document 1 second TTL on (synthetic) error objects

The behavior described here is old (see cache_fetch.c), but has
not been cleanly documented in the docs.

Fixes: #3024

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