Include examples for textual representation of SocketAddrV6

(Examples are from the IPv6 address range reserved for documentation
in IETF RFC 3849.)

Author: nmathewson

library/core/src/net/socket_addr.rs

@@ -106,6 +106,11 @@ pub struct SocketAddrV4 {
 ///   - A colon (`:`)
 ///   - The port, encoded as a decimal integer.
 ///
+/// For example, the string `[2001:db8::413]:443` represents a `SocketAddrV6`
+/// with the address `2001:db8::413` and port `443`.  The string
+/// `[2001:db8::413%612]:443` represents the same address and port, with a
+/// scope identifier of `612`.
+///
 /// Other formats are not accepted.
 ///
 /// [IETF RFC 5952]: https://tools.ietf.org/html/rfc5952#section-6