Documentation: gpio: Reconfiguration with unset direction (uAPI v1)

Update description of reconfiguration rules, adding requirement that a
direction flag be set or the configuration is considered invalid.

Signed-off-by: Kent Gibson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bartosz Golaszewski <[e-mail]>

Documentation/userspace-api/gpio/gpio-handle-set-config-ioctl.rst

···
 
 The same :ref:`gpio-get-linehandle-config-rules` and
 :ref:`gpio-get-linehandle-config-support` that apply when requesting the
-lines also apply when updating the line configuration.
+lines also apply when updating the line configuration, with the additional
+restriction that a direction flag must be set. Requesting an invalid
+configuration, including without a direction flag set, is an error
+(**EINVAL**).
 
 The motivating use case for this command is changing direction of
 bi-directional lines between input and output, but it may be used more