Rollup merge of #25105 - tshepang:doc-addr, r=steveklabnik

 This did not render as intended:

>This is defined in RFC 5737 - 192.0.2.0/24 (TEST-NET-1) - 198.51.100.0/24 (TEST-NET-2) - 203.0.113.0/24 (TEST-NET-3)

vs.

> This is defined in RFC 5737
- 192.0.2.0/24 (TEST-NET-1)
- 198.51.100.0/24 (TEST-NET-2)
- 203.0.113.0/24 (TEST-NET-3)

Author: Manishearth

src/libstd/net/ip.rs

@@ -142,7 +142,8 @@ impl Ipv4Addr {
 
     /// Returns true if this address is in a range designated for documentation.
     ///
-    /// This is defined in RFC 5737
+    /// This is defined in RFC 5737:
+    ///
     /// - 192.0.2.0/24 (TEST-NET-1)
     /// - 198.51.100.0/24 (TEST-NET-2)
     /// - 203.0.113.0/24 (TEST-NET-3)