docs: add "similar to pandas ..." to docstrings

make these more easily findable/understandable
by pandas users

ibis/expr/types/relations.py

@@ -172,6 +172,8 @@ def __contains__(self, name: str) -> bool:
     def cast(self, schema: SupportsSchema) -> Table:
         """Cast the columns of a table.
 
+        Similar to `pandas.DataFrame.astype`.
+
         ::: {.callout-note}
         ## If you need to cast columns to a single type, use [selectors](./selectors.qmd).
         :::
@@ -728,6 +730,8 @@ def group_by(
     ) -> GroupedTable:
         """Create a grouped table expression.
 
+        Similar to SQL's GROUP BY statement, or pandas .groupby() method.
+
         Parameters
         ----------
         by