which unlike HSL Lightness is actually physically meaningful and
also generally speaking much more useful than HSL Lightness.

Change "Lightness" to "Lightness (HSL)" to make it clear that
the "Lightness" in the Colors/Desaturate/Desaturate menu is not the
same as "Lightness" in LAB/LCH.

For completeness add the option to desaturate to "Value (HSV)".

Add links in app/operations/gimpoperationdesaturate.c
to the Wikipedia article with definitions of L/I/V in HSL/HSI/HSV.