DOC: Added sentence to show that unmatched values return -1.

pandas/core/indexes/base.py

@@ -3576,6 +3576,11 @@ def get_loc(self, key, method=None, tolerance=None):
             positions matches the corresponding target values. Missing values
             in the target are marked by -1.
         %(raises_section)s
+        Notes
+        -----
+        Returns -1 for unmatched values, for further explanation see the
+        example below.
+
         Examples
         --------
         >>> index = pd.Index(['c', 'a', 'b'])