Document config parser utility

[babolivier]

We have a utility to run Synapse's config parsing code without starting a homeserver, which can be quite helpful, but it's undocummented and it looks like it was pretty much forgotten about.

We should document it.

The script currently requires the user to provide a config key to print after the config has been parsed, which we sometimes don't care about, so we should make this bit optional.

As a POI, this config check is run like this (in a Synapse venv):

python -m synapse.config read <config key to print> -c <path to config>

More generally, everything after the config key to print is passed to the config parser, so the usage of that bit (which in my example is -c <path to config>) is actually:

usage: __main__.py [-h] [-c CONFIG_FILE] [--keys-directory DIRECTORY] [-D] [--print-pidfile] [--manhole PORT] [-d SQLITE_DATABASE_PATH] [-n] [--enable-registration]

optional arguments:
  -h, --help            show this help message and exit
  -c CONFIG_FILE, --config-path CONFIG_FILE
                        Specify config file. Can be given multiple times and may specify directories containing *.yaml files.
  --keys-directory DIRECTORY
                        Where files such as certs and signing keys are stored when their location is not given explicitly in the config. Defaults to the directory containing the last config file

server:
  -D, --daemonize       Daemonize the homeserver
  --print-pidfile       Print the path to the pidfile just before daemonizing
  --manhole PORT        Turn on the twisted telnet manhole service on the given port.

database:
  -d SQLITE_DATABASE_PATH, --database-path SQLITE_DATABASE_PATH
                        The path to a sqlite database to use.

logging:
  -n, --no-redirect-stdio
                        Do not redirect stdout/stderr to the log

registration:
  --enable-registration
                        Enable registration for new users.

[babolivier]

Note that as of #11145 one can also omit read <config key to print> from the incantation to just get Synapse to try to parse the configuration file to see if it parses fine.