Raise error for invalid order, add note to docs

docs/analysis.rst

@@ -156,6 +156,11 @@ The `~specutils.analysis.moment` function computes moments of any order:
     >>> moment(noisy_gaussian, SpectralRegion(7*u.GHz, 3*u.GHz), order=2) # doctest:+FLOAT_CMP
     <Quantity 0.58586695 GHz2>
 
+By default the moment is calculated along the spectral axis, but any axis can be specified with
+the ``axis`` keyword. Non-spectral-axis moments are calculated using integer pixel values for the
+dispersion and return unitless values. Although they are available, they are perhaps of
+questionable usefulness.
+
 
 Line Widths
 -----------

specutils/analysis/moment.py

@@ -51,8 +51,8 @@ def _compute_moment(spectrum, regions=None, order=0, axis='spectral'):
     """
     This is a helper function for the above `moment()` method.
     """
-    if order is None or order < 0:
-        return None
+    if int(order) != order or order < 0:
+        raise ValueError("Order must be a positive integer.")
 
     if axis == "spectral":
         if isinstance(spectrum, SpectrumCollection):