Slight improvements to the docs, API docs fixes

docs/wcs_utils.rst

@@ -7,9 +7,9 @@ between air and vacuum, as well as a function for calculating the refractive ind
 of air, which is used in the air to vacuum conversions. There are multiple methods
 available for the refractive index calculation, most of which agree with each other
 to a high level of precision. However, the Griesen (2006) equation seems to have a small
-offset from the others (see plot below) and thus in specutils 1.17.0 we have changed
-from using that method as the default to using the Morton (2000) equation, which is
-consistent with the IAU standard.
+offset from the others of about 0.07 Angstrom (at the wavelengths shown in the plot below)
+and thus in specutils 1.17.0 we have changed from using that method as the default to
+using the Morton (2000) equation by default, which is consistent with the IAU standard.
 
 .. image:: air_to_vac_offset.png
    :alt: Results of air to vac conversion at optical wavelengths.
@@ -32,3 +32,4 @@ Reference/API
     :skip: GWCS
     :skip: Shift
     :skip: Tabular1D
+    :skip: SpectralGWCS

specutils/utils/wcs_utils.py

@@ -85,7 +85,8 @@ def refraction_index(wavelength, method='Morton2000', co2=None):
         UV. 'Griesen2006' gives values that are slightly inconsistent with the
         other methods (~0.07 Angstrom difference at visible wavelengths), but it is
         the best option in the FUV due to the mathematical singularities in the others.
-        See https://github.com/astropy/specutils/issues/1162 for more detail.
+        See https://specutils.readthedocs.io/en/latest/wcs_utils.html for more detail, or
+        https://github.com/astropy/specutils/issues/1162 for additional context.
 
     co2 : number, optional
         CO2 concentration in ppm. Only used for method='Ciddor1996'. If not
@@ -162,15 +163,17 @@ def air_to_vac(wavelength, scheme='inversion', method='Morton2000', co2=None,
         How to convert from vacuum to air wavelengths. Options are:
 
             * 'inversion' (default) - result is simply the inversion (1 / n) of the
-            refraction index of air. Griesen et al. (2006) report that the error
-            in naively inverting is less than 10^-9.
+              refraction index of air. Griesen et al. (2006) report that the error
+              in naively inverting is less than 10^-9.
+
             * 'Piskunov' - uses an analytical solution derived by Nikolai Piskunov
-            and used by the Vienna Atomic Line Database (VALD).
+              and used by the Vienna Atomic Line Database (VALD).
+
             * 'iteration' - uses an iterative scheme to invert the index of refraction.
 
     method : str, optional
         Only used if scheme is 'inversion' or 'iteration'. One of the methods
-        in refraction_index(), default is 'Morton2000'
+        in `~specutils.utils.wcs_utils.refraction_index`, default is 'Morton2000'
 
     co2 : number, optional
         Atmospheric CO2 concentration in ppm. Only used if scheme='inversion' and