`to_dataframe` always shows warning if `max_results` is set

[tswast]

to_dataframe always shows a warning that bqstorage_client is set if max_results is set. This warning should not be shown if bqstorage_client is not explicitly set.

Code example

df = bigquery_client.list_rows(
        scalars_table,
        max_results=10,
)

Stack trace

$ pytest tests/system/test_pandas.py::test_list_rows_nullable_scalars_dtypes
========================================= test session starts =========================================
platform darwin -- Python 3.9.5, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /Users/swast/src/python-bigquery
plugins: cov-2.12.1, asyncio-0.15.1
collected 2 items                                                                                     

tests/system/test_pandas.py ..                                                                  [100%]

========================================== warnings summary ===========================================
tests/system/test_pandas.py::test_list_rows_nullable_scalars_dtypes[10]
  /Users/swast/src/python-bigquery/google/cloud/bigquery/table.py:1899: UserWarning: Cannot use bqstorage_client if max_results is set, reverting to fetching data with the REST endpoint.
    if not self._validate_bqstorage(bqstorage_client, create_bqstorage_client):

-- Docs: https://docs.pytest.org/en/stable/warnings.html
==================================== 2 passed, 1 warning in 11.95s ====================================

[plamut]

Simply patching _validate_bqstorage() is troublesome, because there are many paths that directly or indirectly access this helper.

We'll have to move the max_results + BQ Storage client check out of that method and and perform it at all places where user code could call a public method and explicitly pass in a BQ Storage client instance. Everything else seems to result in duplicate warnings/false warnings/no warnings at all.

[tswast]

Yeah, warning just in the public methods probably makes the most sense.

On the plus side, that'd allow the stacklevel argument to warn to work correctly, showing the actually line of the user's code that caused the warning.