Add docs

Signed-off-by: Michael Beardsworth <beardsworth@intrinsic.ai>

include/gz/transport/Discovery.hh

@@ -760,6 +760,8 @@ namespace gz
         this->relayAddrs.push_back(addr);
       }
 
+      // \brief Gets this instance's relay addresses.
+      // \return The list of relay addresses.
       public: std::vector<std::string> RelayAddresses() const
       {
         std::vector<std::string> result;

include/gz/transport/Node.hh

@@ -762,7 +762,17 @@ namespace gz
       public: std::optional<TopicStatistics> TopicStats(
                   const std::string &_topic) const;
 
-      public: void AddGlobalRelay(const std::string& relay_address);
+      /// \brief Adds a unicast relay IP. All nodes in this process will send
+      /// UDP unicast traffic to the address to connect networks when UDP
+      /// multicast traffic is not forwarded.
+      /// It's also possible to use the environment variable GZ_RELAY to add
+      /// relays.
+      /// \param[in] _relayAddress IPv4 address of the relay to add.
+      public: void AddGlobalRelay(const std::string& _relayAddress);
+
+      /// \brief Gets the relay addresses configured for all nodes in this
+      /// process.
+      /// \return The relay addresses.
       public: std::vector<std::string> GlobalRelays() const;
 
       /// \brief Get a pointer to the shared node (singleton shared by all the