DOC: df.to_sql `chunksize` seems to be ignored by default.
#### Location of the documentation

https://dev.pandas.io/docs/reference/api/pandas.DataFrame.to_sql.html


#### Documentation problem

Docs for `chunksize` state that "By default, all rows will be written at once".  However, this seems to only be true if `method="multi"`, even though `method=None` by default.

Experimentally, I notice a substantial speedup if I set `method="multi"` and `chunksize` is either unset or large, but `chunksize` seems to have no effect if I don't set `method`. 

#### Suggested fix for documentation

Documentation for `chunksize` should also reference the `method` argument. For example. we could revise to: 

> chunksize : int, optional
>  Specify the number of rows in each batch to be written at a time, **if batching is enabled via `method`**. By default, all rows will be written at once.

Also, are there many uses where users should avoid `method="multi"`? If not, would it make sense to change the default?