docs: clarify `table_schema` (#383)
docs: clarify `table_schema` (#383)

1. There must be an input dataframe column for every desired output table column.
2. The types given in table_schema must be BigQuery types (strings).

- [x] closes #382 
- [x] tests added / passed
- [x] passes `nox -s blacken lint`

googleapis/python-bigquery-pandas,googleapis/python-bigquery-pandas