Modify default torrc text for PublishServerDescriptor

In the default torrc file, we see:

## By default, Tor will advertise your bridge to users through various
## mechanisms like https://bridges.torproject.org/. If you want to run
## a private bridge, for example because you'll give out your bridge
## address manually to your friends, uncomment this line:
#PublishServerDescriptor 0

However, we've noticed that some default Tor and Orbot bridges are not publishing their descriptors and therefore we aren't getting any metrics from them. This is exactly the use-case that BridgeDistribution none was meant to handle. In general, we recommend that people running private (or default) bridges that aren't meant for BridgeDB to configure their torrc file to have:

PublishServerDescriptor 1
BridgeDistribution none

See also the text in the torrc manpage for PublishServerDescriptor:

       PublishServerDescriptor 0|1|v3|bridge,...
           This option specifies which descriptors Tor will publish when acting as a relay. You
           can choose multiple arguments, separated by commas.

           If this option is set to 0, Tor will not publish its descriptors to any directories.
           (This is useful if you’re testing out your server, or if you’re using a Tor controller
           that handles directory publishing for you.) Otherwise, Tor will publish its descriptors
           of all type(s) specified. The default is "1", which means "if running as a relay or
           bridge, publish descriptors to the appropriate authorities". Other possibilities are
           "v3", meaning "publish as if you’re a relay", and "bridge", meaning "publish as if
           you’re a bridge".

I think we should change the default torrc text to match the manpage and say something to the effect of "if you want to, you can avoid publishing your descriptors, but we recommend that you do and that you set BridgeDistribution none if you don't want your bridge distributed over BridgeDB. That way we can collect metrics."