Updated WHATSNEW with the bug fix information

Author: ivannz

doc/source/whatsnew/v0.19.0.txt

@@ -673,6 +673,7 @@ Performance Improvements
 Bug Fixes
 ~~~~~~~~~
 
+- Bug in ``pandas.parser.parser_trim_buffers()``, which did not update the word vectors (``parser->words``) when stream buffer was shrunk (:issue:`13788`, :issue:`13703`)
 - Bug in ``io.json.json_normalize()``, where non-ascii keys raised an exception (:issue:`13213`)
 - Bug in ``SparseSeries`` with ``MultiIndex`` ``[]`` indexing may raise ``IndexError`` (:issue:`13144`)
 - Bug in ``SparseSeries`` with ``MultiIndex`` ``[]`` indexing result may have normal ``Index`` (:issue:`13144`)