reference to GWE should be added in mf6io.pdf to descriptions of "modelname" in BUY and VSC packages

[rbwinst-usgs]

Describe the bug
The descriptions of the "modelname" variable in the BUY and VSC packages indicate the models should be GWT models. However, they can actually be either GWT or GWE models.

To reproduce
See descriptions of modelname in the BUY and VSC packages on pages 76 and 80 of mf6io.pdf. The two descriptions are identical.
"modelname—name of GWT model used to simulate a species that will be used in the density equation of state.
This name will have no effect if the simulation does not include a GWT model that corresponds to this GWF
model."

Expected behavior
One possible rewording is
modelname—name of GWT or GWE model used to simulate a species that will be used in the density equation of state.
This name will have no effect if the simulation does not include a GWT or GWE model that corresponds to this GWF model.

[emorway-usgs]

Thanks for pointing out this oversight, @rbwinst-usgs. Should be fixed with #2270