docs: add docstrings and news

Author: yucongalicechen

Diff for: news/DO-docstring.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* docstrings for `on_q`, `on_tth`, `on_d`, and `dump` in `diffraction_objects.py`.
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

Diff for: src/diffpy/utils/diffraction_objects.py

@@ -418,12 +418,33 @@ def _get_original_array(self):
             return self.on_d(), "d"
 
     def on_q(self):
+        """Return the tuple of two 1D numpy arrays containing q and y data.
+
+        Returns
+        -------
+        (xarray, yarray) : tuple of ndarray
+            The tuple containing two 1D numpy arrays with q and y data
+        """
         return [self.all_arrays[:, 1], self.all_arrays[:, 0]]
 
     def on_tth(self):
+        """Return the tuple of two 1D numpy arrays containing tth and y data.
+
+        Returns
+        -------
+        (xarray, yarray) : tuple of ndarray
+            The tuple containing two 1D numpy arrays with tth and y data
+        """
         return [self.all_arrays[:, 2], self.all_arrays[:, 0]]
 
     def on_d(self):
+        """Return the tuple of two 1D numpy arrays containing d and y data.
+
+        Returns
+        -------
+        (xarray, yarray) : tuple of ndarray
+            The tuple containing two 1D numpy arrays with d and y data
+        """
         return [self.all_arrays[:, 3], self.all_arrays[:, 0]]
 
     def scale_to(self, target_diff_object, q=None, tth=None, d=None, offset=None):
@@ -507,6 +528,16 @@ def on_xtype(self, xtype):
             raise ValueError(_xtype_wmsg(xtype))
 
     def dump(self, filepath, xtype=None):
+        """Dump the xarray and yarray of the diffraction object to a two-column
+        file, with the associated information included in the header.
+
+        Parameters
+        ----------
+        filepath : str
+            The filepath where the diffraction object will be dumped
+        xtype : str, optional, default is q
+            The type of quantity for the independent variable chosen from {*XQUANTITIES, }
+        """
         if xtype is None:
             xtype = "q"
         if xtype in QQUANTITIES: