Improve documentation of st_graticule()

[clauswilke]

I think it would be helpful if the documentation of st_graticule() could be improved a bit. Specifically, the lon and lat parameters can be a bit confusing. If I understand correctly, they are interpreted in the CRS provided as datum, so they are only longitude and latitude if that CRS is a long-lat CRS. Stating this in even a single sentence would be helpful.

Specifically, currently the documentation states:

• lon: numeric; degrees east for the meridians
• lat: numeric; degrees north for the parallels

I believe this is not true in general, for any arbitrary datum, and also not if datum = NULL.