Fix docs incorrectly stating primary keys default to unsigned

The documentation claimed primary keys are unsigned by default, but the
actual code defaults to signed. Unsigned primary keys require either
passing 'signed' => false or enabling the Migrations.unsigned_primary_keys
feature flag.

Author: dereuromark

docs/en/index.rst

@@ -841,6 +841,8 @@ pass them to the method::
     // This one with the "default" connection
     $migrate = $migrations->migrate(['connection' => 'default']);
 
+.. _feature-flags:
+
 Feature Flags
 =============
 

docs/en/writing-migrations.rst

@@ -549,13 +549,14 @@ comment    MySQL, Postgres  set a text comment on the table
 collation  MySQL, SqlServer set the table collation *(defaults to database collation)*
 row_format MySQL            set the table row format
 engine     MySQL            define table engine *(defaults to ``InnoDB``)*
-signed     MySQL            whether the primary key is ``signed``  *(defaults to ``false``)*
+signed     MySQL            whether the primary key is ``signed``  *(defaults to ``true``)*
 limit      MySQL            set the maximum length for the primary key
 ========== ================ ===========
 
-By default, the primary key is ``unsigned``.
-To simply set it to be signed just pass ``signed`` option with a ``true``
-value::
+By default, the primary key is ``signed``.
+To set it to be unsigned, pass the ``signed`` option with a ``false``
+value, or enable the ``Migrations.unsigned_primary_keys`` feature flag
+(see :ref:`feature-flags`)::
 
     <?php