add docstring for Sockets.DNSError (#34189)

Author: AtsushiSakai

stdlib/Sockets/docs/src/index.md

@@ -10,6 +10,7 @@ Sockets.getaddrinfo
 Sockets.getipaddr
 Sockets.getipaddrs
 Sockets.getalladdrinfo
+Sockets.DNSError
 Sockets.getnameinfo
 Sockets.getsockname
 Sockets.getpeername

stdlib/Sockets/src/addrinfo.jl

@@ -1,5 +1,12 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
+"""
+    DNSError
+
+The type of exception thrown when an error occurs in DNS lookup.
+The `host` field indicates the host URL string.
+The `code` field indicates the error code based on libuv.
+"""
 struct DNSError <: Exception
     host::String
     code::Int32