bpo-41205: Document Decimal power 0 to the 0 (pythonGH-21386)

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>
(cherry picked from commit 10e4664)

Co-authored-by: Srinivas Reddy Thatiparthy (శ్రీనివాస్  రెడ్డి తాటిపర్తి) <thatiparthysreenivas@gmail.com>

Doc/library/decimal.rst

@@ -1357,6 +1357,9 @@ In addition to the three supplied contexts, new contexts can be created with the
       The rounding mode of the context is used. Results are always correctly-rounded
       in the Python version.
 
+      ``Decimal(0) ** Decimal(0)`` results in ``InvalidOperation``, and if ``InvalidOperation``
+      is not trapped, then results in ``Decimal('NaN')``.
+
       .. versionchanged:: 3.3
          The C module computes :meth:`power` in terms of the correctly-rounded
          :meth:`exp` and :meth:`ln` functions. The result is well-defined but