DOC: behavior when slicing with missing bounds

closes #16917

Author: toobaz

doc/source/indexing.rst

@@ -78,8 +78,12 @@ of multi-axis indexing.
     *label* of the index. This use is **not** an integer position along the
     index)
   - A list or array of labels ``['a', 'b', 'c']``
-  - A slice object with labels ``'a':'f'``, (note that contrary to usual python
-    slices, **both** the start and the stop are included!)
+  - A slice object with labels ``'a':'f'``: note that contrary to usual python
+    slices, **both** the start and the stop are included, when present in the
+    index! If at least one of the two is absent but the index elements are
+    sorted (and can be compared against start and stop labels), then slicing
+    will still work as expected, that is, selecting labels which rank between
+    ``'a'`` and ``'f'``.
   - A boolean array
   - A ``callable`` function with one argument (the calling Series, DataFrame or Panel) and
     that returns valid output for indexing (one of the above)
@@ -330,13 +334,18 @@ Selection By Label
      dfl.loc['20130102':'20130104']
 
 pandas provides a suite of methods in order to have **purely label based indexing**. This is a strict inclusion based protocol.
-**At least 1** of the labels for which you ask, must be in the index or a ``KeyError`` will be raised! When slicing, the start bound is *included*, **AND** the stop bound is *included*. Integers are valid labels, but they refer to the label **and not the position**.
+**At least 1** of the labels for which you ask, must be in the index or a ``KeyError`` will be raised! When slicing, both the start bound **AND** the stop bound are *included*, if present in the index. Integers are valid labels, but they refer to the label **and not the position**.
 
 The ``.loc`` attribute is the primary access method. The following are valid inputs:
 
 - A single label, e.g. ``5`` or ``'a'``, (note that ``5`` is interpreted as a *label* of the index. This use is **not** an integer position along the index)
 - A list or array of labels ``['a', 'b', 'c']``
-- A slice object with labels ``'a':'f'`` (note that contrary to usual python slices, **both** the start and the stop are included!)
+- A slice object with labels ``'a':'f'``: note that contrary to usual python
+  slices, **both** the start and the stop are included, when present in the
+  index! If at least one of the two is absent but the index elements are sorted
+  (and can be compared against start and stop labels), then slicing will still
+  work as expected, that is selecting labels which rank between ``'a'`` and
+  ``'f'``.
 - A boolean array
 - A ``callable``, see :ref:`Selection By Callable <indexing.callable>`