doc: Fix typo and clarify that DoS options are relay only

Tor has three built-in mitigation options that can be individually

enabled/disabled and fine-tuned, but by default Tor directory authorities will

define reasonable values for relays and no explicit configuration is required

to make use of these protections.  The mitigations are:

to make use of these protections. The mitigations take place at relays,

and are as follows:

  1. If a single client address makes too many concurrent connections (this is

     configurable via DoSConnectionMaxConcurrentCount), hang up on further

     Tor2Web client), ignore the request.

These defenses can be manually controlled by torrc options, but relays will

also take guidance from consensus parameters, so there's no need to configure

anything manually. In doubt, do not change those values.

also take guidance from consensus parameters using these same names, so there's

no need to configure anything manually. In doubt, do not change those values.

The values set by the consensus, if any, can be found here:

https://consensus-health.torproject.org/#consensusparams

If any of the DoS mitigations are enabled, an heartbeat message will appear in

If any of the DoS mitigations are enabled, a heartbeat message will appear in

your log at NOTICE level which looks like:

    DoS mitigation since startup: 429042 circuits rejected, 17 marked addresses.