DOC: release note for pandas-dev#9607, as_blocks copy arg

Author: jreback

doc/source/whatsnew/v0.17.0.txt

@@ -31,6 +31,8 @@ New features
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
 
+- ``.as_blocks`` will now take a ``copy`` optional argument to return a copy of the data, default is to copy (no change in behavior from prior versions), (:issue:`9607`)
+
 .. _whatsnew_0170.api:
 
 Backwards incompatible API changes