This is mostly a documentation change, but also adds a default value for
SYNAPSE_CONFIG_PATH, so that running from the generated config is the default,
and will Just Work provided your config is in the right place.
When running under docker, we want to use docker's own logging stuff rather
than losing the logs somewhere on the container's filesystem, so let's use log
configs that spit logs out to stdout instead.
We don't want to generate any missing configs when running from a precanned
config.
(There's a strong argument that we don't want to do this at all, since
generating a new signing key on each invocation sounds disasterous, but I don't
fancy unpicking that for now.)