chore(bigquery): add comment about why the conversion is needed

Author: cpcloud

ibis/backends/bigquery/__init__.py

@@ -697,6 +697,11 @@ def _to_sqlglot(
         if not geocols:
             return query
 
+        # if there are any geospatial columns, we have to convert them to WKB,
+        # so interactive mode knows how to display them
+        #
+        # by default bigquery returns data to python as WKT, and there's really
+        # no point in supporting both if we don't need to.
         quoted = self.compiler.quoted
         return sg.select(
             sge.Star(