Update "ITU-R BT.2020" colourspace docstrings.
@@ -91,6 +91,12 @@ RGB_COLOURSPACE_BT2020 = RGB_Colourspace(
RGB_COLOURSPACE_BT2020.__doc__ = """
*ITU-R BT.2020* colourspace.
+The wavelength of the *ITU-R BT.2020* primary colours are:
+
+- 630nm for the red primary colour
+- 532nm for the green primary colour
+- 467nm for the blue primary colour.
+
References
----------
:cite:`InternationalTelecommunicationUnion2015h`
