Implementation of ``BlockDevice::get_default_instance()`` does not match the document description

/** Return the default block device
     *
     * Returns the default block device based on the configuration JSON.
     * Use the components in target.json or application config to change
     * the default block device.
     *
     * An application can override all target settings by implementing
     * BlockDevice::get_default_instance() - the default
     * definition is weak, and calls get_target_default_instance().
    */
    static BlockDevice *get_default_instance();

The document in BlockDevice said this function will call get_target_default_instance, but the implementation just calls the default constructor, which not match the document, and the subclass of BlockDevice e.g. QSPIFBlockDevice has no static member called get_target_default_instance

Thank you for your feedback. Please see https://github.com/ARMmbed/mbed-os/pull/13161