DOC: Polynomial domain clarification

This commit adds a note about the `coef` array in Polynomial
being stored in the domain scaled according to the `window`
and `domain` parameters, and not, as a np.polyfit user
may expect, in the usual data coordinates.

This is a followup to issue numpy#9533.

numpy/polynomial/_polybase.py

@@ -805,6 +805,10 @@ def fit(cls, x, y, deg, domain=None, rcond=None, full=False, w=None,
         specified and this will often result in a superior fit with less
         chance of ill conditioning.
 
+        Please note that ``Polynomial.coef`` are coefficients in the scaled
+        domain, and that to get coefficients from the data domain, you should
+        use ``Polynomial.convert().coef``.
+
         Parameters
         ----------
         x : array_like, shape (M,)

numpy/polynomial/polynomial.py

@@ -1443,6 +1443,10 @@ class Polynomial(ABCPolyBase):
     '+', '-', '*', '//', '%', 'divmod', '**', and '()' as well as the
     attributes and methods listed in the `ABCPolyBase` documentation.
 
+    Please note that ``Polynomial.coef`` are coefficients in the scaled
+    domain, and that to get coefficients from the data domain, you
+    should use ``Polynomial.convert().coef``.
+
     Parameters
     ----------
     coef : array_like