DOC: Update whatsnew about NDFrame._dir_additions enhancements (pandas-dev#16326, pandas-dev#18587)

Author: Martin Journois

doc/source/whatsnew/v0.22.0.txt

@@ -163,7 +163,8 @@ Other Enhancements
 - :func:`pandas.read_clipboard` updated to use qtpy, falling back to PyQt5 and then PyQt4, adding compatibility with Python3 and multiple python-qt bindings (:issue:`17722`)
 - Improved wording of ``ValueError`` raised in :func:`read_csv` when the ``usecols`` argument cannot match all columns. (:issue:`17301`)
 - :func:`DataFrame.corrwith` now silently drops non-numeric columns when passed a Series. Before, an exception was raised (:issue:`18570`).
-
+- :func:`NDFrame._dir_additions` (tab completion) also returns identifiers in the first level of a :func:`MultiIndex`. (:issue:`16326`)
+- :func:`NDFrame._dir_additions` (tab completion) limits to 100 values, for better performance. (:issue:`18587`)
 
 .. _whatsnew_0220.api_breaking: