mark custom primary keys as experimental for now

I'll revert this change once custom primary keys are supported fully in Piccolo admin

Author: dantownsend

docs/src/piccolo/schema/defining.rst

@@ -24,12 +24,13 @@ For a full list of columns, see :ref:`ColumnTypes`.
 -------------------------------------------------------------------------------
 
 Primary Key
----------------
+-----------
 
-You can specify your ``PrimaryKey`` with any column type by passing ``primary_key`` to the ``Column``.
+Piccolo tables are automatically given a primary key column called ``id``,
+which is an auto incrementing integer.
 
-It is used to uniquely identify a row, and is referenced by ``ForeignKey``
-columns on other tables.
+There is currently experimental support for specifying a custom primary key
+column. For example:
 
 .. code-block:: python
 
@@ -42,22 +43,6 @@ columns on other tables.
         id = UUID(primary_key=True)
         name = Varchar(length=100)
 
-If you don't specify a ``PrimaryKey``, the table is automatically given a ``PrimaryKey`` column called ``id``, which
-is an auto incrementing integer.
-
-This is equivalent to:
-
-.. code-block:: python
-
-    # tables.py
-    from piccolo.table import Table
-    from piccolo.columns import Serial, Varchar
-
-
-    class Band(Table):
-        id = Serial(primary_key=True)
-        name = Varchar(length=100)
-
 -------------------------------------------------------------------------------
 
 Tablename