Fix the Docstring. Remove unnecessary doctest directive, and add a note on ValueError Exception raised.

Author: orsenthil

Doc/library/ipaddress.rst

@@ -721,9 +721,12 @@ dictionaries.
    .. method:: next_network(next_prefix=None)
 
       Finds the next closest network of prefix size *next_prefix*.  If
-      *next_prefix=None*, then the current network prefix will be used.  #doctest: +NORMALIZE_WHITESPACE
+      *next_prefix=None*, then the current network prefix will be used.
       Returns a single network object.
 
+      Raises :exc:`ValueError` if *next_prefix* is out of range or no further
+      network of the requested size exists.
+
          >>> IPv4Network('192.0.2.0/24').next_network()
          IPv4Network('192.0.3.0/24')
          >>> IPv4Network('192.0.2.0/24').next_network(next_prefix=25)