[1.6.x] Fixed #21427 -- Clearly state integer field value ranges in docs

Added an explicit mention of the exact value ranges for integer type
fields that are safe in all databases supported by Django. Also, put
all value numbers inside double ticks.

Backport of b6a6cf4ab7 from master.

docs/ref/models/fields.txt

@@ -357,8 +357,9 @@ otherwise. See :ref:`automatic-primary-key-fields`.
 .. class:: BigIntegerField([**options])
 
 A 64 bit integer, much like an :class:`IntegerField` except that it is
-guaranteed to fit numbers from -9223372036854775808 to 9223372036854775807. The
+guaranteed to fit numbers from ``-9223372036854775808`` to
-default form widget for this field is a :class:`~django.forms.TextInput`.
+``9223372036854775807``. The default form widget for this field is a
+:class:`~django.forms.TextInput`.
 
 ``BinaryField``
 -------------------
@@ -496,8 +497,8 @@ A fixed-precision decimal number, represented in Python by a
 
     The number of decimal places to store with the number.
 
-For example, to store numbers up to 999 with a resolution of 2 decimal places,
+For example, to store numbers up to ``999`` with a resolution of 2 decimal
-you'd use::
+places, you'd use::
 
     models.DecimalField(..., max_digits=5, decimal_places=2)
 
@@ -837,7 +838,8 @@ length using the :attr:`~CharField.max_length` argument.
 
 .. class:: IntegerField([**options])
 
-An integer. The default form widget for this field is a
+An integer. Values from ``-2147483648`` to ``2147483647`` are safe in all
+databases supported by Django. The default form widget for this field is a
 :class:`~django.forms.TextInput`.
 
 ``IPAddressField``
@@ -894,7 +896,8 @@ widget for this field is a :class:`~django.forms.NullBooleanSelect`.
 .. class:: PositiveIntegerField([**options])
 
 Like an :class:`IntegerField`, but must be either positive or zero (``0``).
-The value ``0`` is accepted for backward compatibility reasons.
+Values from ``0`` to ``2147483647`` are safe in all databases supported by
+Django. The value ``0`` is accepted for backward compatibility reasons.
 
 ``PositiveSmallIntegerField``
 -----------------------------
@@ -902,8 +905,8 @@ The value ``0`` is accepted for backward compatibility reasons.
 .. class:: PositiveSmallIntegerField([**options])
 
 Like a :class:`PositiveIntegerField`, but only allows values under a certain
-(database-dependent) point. Values up to 32767 are safe in all databases
+(database-dependent) point. Values from ``0`` to ``32767`` are safe in all
-supported by Django.
+databases supported by Django.
 
 ``SlugField``
 -------------
@@ -931,8 +934,8 @@ of some other value.  You can do this automatically in the admin using
 .. class:: SmallIntegerField([**options])
 
 Like an :class:`IntegerField`, but only allows values under a certain
-(database-dependent) point. Values from -32768 to 32767 are safe in all databases
+(database-dependent) point. Values from ``-32768`` to ``32767``` are safe in all
-supported by Django.
+databases supported by Django.
 
 ``TextField``
 -------------