DOC: add whats new entry for addressing pandas-dev#14855

Author: alexandercbooth

doc/source/whatsnew/v0.19.2.txt

@@ -80,3 +80,6 @@ Bug Fixes
 - Explicit check in ``to_stata`` and ``StataWriter`` for out-of-range values when writing doubles (:issue:`14618`)
 
 - Bug in ``unstack()`` if called with a list of column(s) as an argument, regardless of the dtypes of all columns, they get coerced to ``object`` (:issue:`11847`)
+
+- Bug in ``pd.scatter_matrix()`` could accept either ``color`` or ``c``, but
+  not both (:issue:`14855`)