Document network.public-ip-endpoints in example config

Update example.config.toml: refresh the public-ipv4/public-ipv6
comment to mention the new multi-endpoint detection (no longer
solely ifconfig.co), and document the new
network.public-ip-endpoints option with the current default values.

example.config.toml

 prefer-ip = "prefer-ipv6"
 
 # Public IP addresses of this server. Used by 'mtg access' to generate
-# proxy links and by 'mtg doctor' to validate SNI-DNS match.
-# If not set, mtg tries to detect them automatically via ifconfig.co.
-# Set these if ifconfig.co is unreachable from your server.
+# proxy links and by 'mtg doctor' / proxy startup to validate SNI-DNS match.
+# If not set, mtg tries to detect them automatically by querying the public
+# HTTPS endpoints listed in network.public-ip-endpoints (see below).
+# Set these explicitly if those endpoints are unreachable from your server.
 # public-ipv4 = "1.2.3.4"
 # public-ipv6 = "2001:db8::1"
 
     # "socks5://user:password@host:port"
 ]
 
+# HTTPS endpoints used to discover this server's public IPv4/IPv6 when
+# public-ipv4 / public-ipv6 are not set. Each must return the client's public
+# IP as a single address in the plain-text response body. mtg tries them in
+# order and uses the first that succeeds. The default is shown below; setting
+# this option overrides the default entirely.
+# public-ip-endpoints = [
+#     "https://ifconfig.co",
+#     "https://icanhazip.com",
+#     "https://ifconfig.me",
+# ]
+
 # network timeouts define different settings for timeouts. tcp timeout
 # define a global timeout on establishing of network connections. idle
 # means a timeout on pumping data between sockset when nothing is