Corrected field and model check messages in docs.

Follow up to 72d04e03855536cf36090aedbb7aa8f1c4c8b443.

docs/ref/checks.txt

@@ -172,7 +172,7 @@ Model fields
 * **fields.E003**: ``pk`` is a reserved word that cannot be used as a field
   name.
 * **fields.E004**: ``choices`` must be an iterable (e.g., a list or tuple).
-* **fields.E005**: ``choices`` must be an iterable returning ``(actual value,
+* **fields.E005**: ``choices`` must be an iterable containing ``(actual value,
   human readable name)`` tuples.
 * **fields.E006**: ``db_index`` must be ``None``, ``True`` or ``False``.
 * **fields.E007**: Primary keys must not have ``null=True``.
@@ -191,12 +191,12 @@ Model fields
 * **fields.E130**: ``DecimalField``\s must define a ``decimal_places`` attribute.
 * **fields.E131**: ``decimal_places`` must be a non-negative integer.
 * **fields.E132**: ``DecimalField``\s must define a ``max_digits`` attribute.
-* **fields.E133**: ``max_digits`` must be a non-negative integer.
+* **fields.E133**: ``max_digits`` must be a positive integer.
 * **fields.E134**: ``max_digits`` must be greater or equal to ``decimal_places``.
 * **fields.E140**: ``FilePathField``\s must have either ``allow_files`` or
   ``allow_folders`` set to True.
-* **fields.E150**: ``GenericIPAddressField``\s cannot accept blank values if
-  null values are not allowed, as blank values are stored as nulls.
+* **fields.E150**: ``GenericIPAddressField``\s cannot have ``blank=True`` if
+  ``null=False``, as blank values are stored as nulls.
 * **fields.E160**: The options ``auto_now``, ``auto_now_add``, and ``default``
   are mutually exclusive. Only one of these options may be present.
 * **fields.W161**: Fixed default value provided.
@@ -247,17 +247,16 @@ Related fields
   either not installed, or is abstract.
 * **fields.E301**: Field defines a relation with the model
   ``<app_label>.<model>`` which has been swapped out.
-* **fields.E302**: Accessor for field ``<app_label>.<model>.<field name>``
-  clashes with field ``<app_label>.<model>.<field name>``.
-* **fields.E303**: Reverse query name for field
-  ``<app_label>.<model>.<field name>`` clashes with field
-  ``<app_label>.<model>.<field name>``.
-* **fields.E304**: Field name ``<app_label>.<model>.<field name>`` clashes with
-  accessor for ``<app_label>.<model>.<field name>``.
-* **fields.E305**: Field name ``<app_label>.<model>.<field name>`` clashes with
-  reverse query name for ``<app_label>.<model>.<field name>``.
-* **fields.E306**: Related name must be a valid Python identifier or end with
-  a ``'+'``.
+* **fields.E302**: Reverse accessor for ``<app_label>.<model>.<field name>``
+  clashes with field name ``<app_label>.<model>.<field name>``.
+* **fields.E303**: Reverse query name for ``<app_label>.<model>.<field name>``
+  clashes with field name ``<app_label>.<model>.<field name>``.
+* **fields.E304**: Reverse accessor for ``<app_label>.<model>.<field name>``
+  clashes with reverse accessor for ``<app_label>.<model>.<field name>``.
+* **fields.E305**: Reverse query name for ``<app_label>.<model>.<field name>``
+  clashes with reverse query name for ``<app_label>.<model>.<field name>``.
+* **fields.E306**: The name ``<name>`` is invalid ``related_name`` for field
+  ``<model>.<field name>``.
 * **fields.E307**: The field ``<app label>.<model>.<field name>`` was declared
   with a lazy reference to ``<app label>.<model>``, but app ``<app label>``
   isn't installed or doesn't provide model ``<model>``.
@@ -324,8 +323,8 @@ Models
   sets ``primary_key=True``.
 * **models.E005**: The field ``<field name>`` from parent model ``<model>``
   clashes with the field ``<field name>`` from parent model ``<model>``.
-* **models.E006**: The field clashes with the field ``<field name>`` from model
-  ``<model>``.
+* **models.E006**: The field ``<field name>`` clashes with the field
+  ``<field name>`` from model ``<model>``.
 * **models.E007**: Field ``<field name>`` has column name ``<column name>``
   that is used by another field.
 * **models.E008**: ``index_together`` must be a list or tuple.