Add deprecation doc

Author: jseabold

doc/source/whatsnew/v1.2.0.rst

@@ -510,6 +510,7 @@ Deprecations
 - Deprecated :meth:`Index.asi8` for :class:`Index` subclasses other than :class:`.DatetimeIndex`, :class:`.TimedeltaIndex`, and :class:`PeriodIndex` (:issue:`37877`)
 - The ``inplace`` parameter of :meth:`Categorical.remove_unused_categories` is deprecated and will be removed in a future version (:issue:`37643`)
 - The ``null_counts`` parameter of :meth:`DataFrame.info` is deprecated and replaced by ``show_counts``. It will be removed in a future version (:issue:`37999`)
+- Deprecated default keyword argument of ``observed=False`` in :~meth:`DataFrame.groupby` and :~meth:`DataFrame.pivot_table` (:issue:`17594`)
 
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