[1.6.x] Fixed #22145 -- Clarified docs on Field.blank and Field.null options.

Thanks skipjack at peru.com for the suggestion.

Backport of 3390550f6f from master

docs/ref/models/fields.txt

@@ -39,19 +39,17 @@ The following arguments are available to all field types. All are optional.
 If ``True``, Django will store empty values as ``NULL`` in the database. Default
 is ``False``.
 
-Note that empty string values will always get stored as empty strings, not as
-``NULL``. Only use ``null=True`` for non-string fields such as integers,
-booleans and dates. For both types of fields, you will also need to set
-``blank=True`` if you wish to permit empty values in forms, as the
-:attr:`~Field.null` parameter only affects database storage (see
-:attr:`~Field.blank`).
-
 Avoid using :attr:`~Field.null` on string-based fields such as
-:class:`CharField` and :class:`TextField` unless you have an excellent reason.
-If a string-based field has ``null=True``, that means it has two possible values
-for "no data": ``NULL``, and the empty string. In most cases, it's redundant to
-have two possible values for "no data;" Django convention is to use the empty
-string, not ``NULL``.
+:class:`CharField` and :class:`TextField` because empty string values will
+always be stored as empty strings, not as ``NULL``. If a string-based field has
+``null=True``, that means it has two possible values for "no data": ``NULL``,
+and the empty string. In most cases, it's redundant to have two possible values
+for "no data;" the Django convention is to use the empty string, not ``NULL``.
+
+For both string-based and non-string-based fields, you will also need to
+set ``blank=True`` if you wish to permit empty values in forms, as the
+:attr:`~Field.null` parameter only affects database storage
+(see :attr:`~Field.blank`).
 
 .. note::