Fixed #18046 - Documented than an index is created by default for ForeignKeys;...

This sort of reference can be useful when resolving circular import

dependencies between two applications.

A database index is automatically created on the ``ForeignKey``. You can

disable this by setting :attr:`~Field.db_index` to ``False``.  You may want to

avoid the overhead of an index if you are creating a foreign key for

consistency rather than joins, or if you will be creating an alternative index

like a partial or multiple column index.

Database Representation

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