DOC: Add a release note for fully annotating the main numpy namespace

doc/release/upcoming_changes/20217.improvements.rst

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+Annotating the main Numpy namespace
+--------------------------------------
+Starting from the 1.20 release, PEP 484 type annotations have been included
+for parts of the NumPy library; annotating the remaining functions being a
+work in progress. With the release of 1.22 this process has been completed for
+the main NumPy namespace, which is now fully annotated.
+
+Besides the main namespace, a limited number of sub-packages contain
+annotations as well. This includes, among others, `numpy.testing`,
+`numpy.linalg` and `numpy.random` (available since 1.21).