Clarify docs on what IPv4 octal addresses are

The way octal literals are written in IP addresses differs from the way
they are written in Rust code, so the way that octal/hex literals in IPs
are written is explictly mentioned.

library/std/src/net/ip.rs

@@ -59,7 +59,8 @@ pub enum IpAddr {
 ///
 /// `Ipv4Addr` provides a [`FromStr`] implementation. The four octets are in decimal
 /// notation, divided by `.` (this is called "dot-decimal notation").
-/// Notably, octal numbers and hexadecimal numbers are not allowed per [IETF RFC 6943].
+/// Notably, octal numbers (which are indicated with a leading `0`) and hexadecimal numbers (which
+/// are indicated with a leading `0x`) are not allowed per [IETF RFC 6943].
 ///
 /// [IETF RFC 6943]: https://tools.ietf.org/html/rfc6943#section-3.1.1
 /// [`FromStr`]: crate::str::FromStr
@@ -72,6 +73,8 @@ pub enum IpAddr {
 /// let localhost = Ipv4Addr::new(127, 0, 0, 1);
 /// assert_eq!("127.0.0.1".parse(), Ok(localhost));
 /// assert_eq!(localhost.is_loopback(), true);
+/// assert!("012.004.002.000".parse::<Ipv4Addr>().is_err()); // all octets are in octal
+/// assert!("0xcb.0x0.0x71.0x00".parse::<Ipv4Addr>().is_err()); // all octets are in hex
 /// ```
 #[derive(Copy)]
 #[stable(feature = "rust1", since = "1.0.0")]