core/doc: correct Writer::delimiter guarantee

Previously, the docs said it only ever writes at most 1 byte. However,
if delimiter is called while quoting, then it may write the end of the quote
as well.

PR #192

csv-core/src/writer.rs

@@ -313,7 +313,8 @@ impl Writer {
  /// a field delimiter, then nothing is written to `output`
  /// and `WriteResult::OutputFull` is returned. Otherwise,
  /// `WriteResult::InputEmpty` is returned along with the number of bytes
- /// written to `output` (which is always `1`).
+ /// written to `output` (which is `1` in case of an unquoted
+ /// field, or `2` in case of an end quote and a field separator).
  pub fn delimiter(
  &mut self,
  mut output: &mut [u8],