Document the default CPT for GMT remote datasets

pygmt/datasets/earth_age.py

@@ -26,6 +26,13 @@ def load_earth_age(resolution="01d", region=None, registration=None):
     registration type (**p** for pixel registration or **g** for gridline
     registration).
 
+    The default CPT for this dataset is *@earth_age.cpt*. It's implicitly used
+    when passing in the file name of the dataset to any grid plotting function
+    if no CPT is explicitly specified. When the dataset is loaded and plotted
+    as an :class:`xarray.DataArray` object, the default CPT is ignored and
+    GMT's default CPT (*turbo*) is used. To use the dataset-specific CPT, you
+    need to explicitly set ``cpt="@earth_age.cpt"``.
+
     Refer to :gmt-datasets:`earth-age.html` for more details.
 
     Parameters