Feature request: Writer::terminate_record

#### What version of the `csv` crate are you using?

1.2.0

#### Briefly describe the question, bug or feature request.

`csv::Writer::write_field` suggests to use `write_record` with empty iterator to terminate a record:

> Note that if this API is used, write_record should be called with an empty iterator to write a record terminator.

However, using `write_record` this way requires a clumsy turbofish, as shown in the documentation:

> `wtr.write_record(None::<&[u8]>)?;`

It is even more confusing if user tries to go the `wtr.write_recrord::(...);` route instead.

I suggest to add a dedicated method like `terminate_record` that is equivalent to `write_record(None::<&[u8]>)`, then update documentation of `write_field` to use that method.