Issue #83: improve babl_space_get_rgb_luminance() docs.

Issue #83: improve babl_space_get_rgb_luminance() docs.

One of GIMP layer modes was using wrongfully these coefficients on
non-linear data (see gimp#3471). It was not clear from babl docs whether
babl was returning wrong coefficients for non-linear spaces, or whether
we were supposed to only ever use these coefficients on linear data.
This is the second case. So let's make the in-code docs clearer.