DOC: Add cookbook entry for triangular correlation matrix (GH22840) (pandas-dev#23032)

Author: Daniel Saxton

doc/source/cookbook.rst

@@ -1226,6 +1226,17 @@ Computation
 Correlation
 ***********
 
+Often it's useful to obtain the lower (or upper) triangular form of a correlation matrix calculated from :func:`DataFrame.corr`.  This can be achieved by passing a boolean mask to ``where`` as follows:
+
+.. ipython:: python
+
+    df = pd.DataFrame(np.random.random(size=(100, 5)))
+
+    corr_mat = df.corr()
+    mask = np.tril(np.ones_like(corr_mat, dtype=np.bool), k=-1)
+
+    corr_mat.where(mask)
+
 The `method` argument within `DataFrame.corr` can accept a callable in addition to the named correlation types.  Here we compute the `distance correlation <https://en.wikipedia.org/wiki/Distance_correlation>`__ matrix for a `DataFrame` object.
 
 .. code-block:: python