docs: improve Table.columns docstring

Before it could be interpreted that this
returned Column objects. But it just returns
string names of the columns.

Per review comment, I'm not putting in the full
`list[str]` type annotation because it should get
rendered in quartodoc, it just isn't.

ibis/expr/types/relations.py

@@ -677,7 +677,7 @@ def _ensure_expr(self, expr):
 
     @property
     def columns(self) -> list[str]:
-        """The list of columns in this table.
+        """The list of column names in this table.
 
         Examples
         --------