docs: iio: ad7380: fix supply for ad7380-4

ad7380-4 is the only device from ad738x family that doesn't have an
internal reference. Moreover it's external reference is called REFIN in
the datasheet while all other use REFIO as an optional external
reference. Update documentation to highlight this.

Fixes: 3e82dfc82f38 ("docs: iio: new docs for ad7380 driver")
Reviewed-by: David Lechner <[e-mail]>
Signed-off-by: Julien Stephan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/iio/ad7380.rst

···
 Reference voltage
 -----------------
 
-2 possible reference voltage sources are supported:
+ad7380-4
+~~~~~~~~
+
+ad7380-4 supports only an external reference voltage (2.5V to 3.3V). It must be
+declared in the device tree as ``refin-supply``.
+
+All other devices from ad738x family
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+All other devices from ad738x support 2 possible reference voltage sources:
 
 - Internal reference (2.5V)
 - External reference (2.5V to 3.3V)
 
 The source is determined by the device tree. If ``refio-supply`` is present,
-then the external reference is used, else the internal reference is used.
+then it is used as external reference, else the internal reference is used.
 
 Oversampling and resolution boost
 ---------------------------------