Fixed #22193 -- Made hint a truly optional arugment on check messages.

Thanks to Thomas Güttler for the suggestion.

django/core/checks/messages.py

@@ -15,7 +15,7 @@ CRITICAL = 50
 @python_2_unicode_compatible
 class CheckMessage(object):
 
-    def __init__(self, level, msg, hint, obj=None, id=None):
+    def __init__(self, level, msg, hint=None, obj=None, id=None):
         assert isinstance(level, int), "The first argument should be level."
         self.level = level
         self.msg = msg

docs/topics/checks.txt

@@ -78,12 +78,8 @@ Constructor arguments are:
 
 ``hint``
     A single-line string providing a hint for fixing the problem. If no hint
-    can be provided, or the hint is self-evident from the error message, a
+    can be provided, or the hint is self-evident from the error message, the
-    value of ``None`` can be used::
+    hint can be omitted, or a value of ``None`` can be used.
-
-        Error('error message') # Will not work.
-        Error('error message', None) # Good
-        Error('error message', hint=None) # Better
 
 ``obj``
     Optional. An object providing context for the message (for example, the