DOC: add internal links to "Endpoints are inclusive" section (#27131)

Author: pilkibun

doc/source/user_guide/advanced.rst

@@ -1108,6 +1108,8 @@ the :meth:`~Index.is_unique` attribute.
    weakly_monotonic.is_monotonic_increasing
    weakly_monotonic.is_monotonic_increasing & weakly_monotonic.is_unique
 
+.. _advanced.endpoints_are_inclusive:
+
 Endpoints are inclusive
 ~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -1137,7 +1139,7 @@ index can be somewhat complicated. For example, the following does not work:
     s.loc['c':'e' + 1]
 
 A very common use case is to limit a time series to start and end at two
-specific dates. To enable this, we made the design to make label-based
+specific dates. To enable this, we made the design choice to make label-based
 slicing include both endpoints:
 
 .. ipython:: python

doc/source/user_guide/indexing.rst

@@ -61,8 +61,8 @@ of multi-axis indexing.
     * 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, when present in the
-      index! See :ref:`Slicing with labels
-      <indexing.slicing_with_labels>`.).
+      index! See :ref:`Slicing with labels <indexing.slicing_with_labels>`
+      and :ref:`Endpoints are inclusive <advanced.endpoints_are_inclusive>`.)
     * A boolean array
     * A ``callable`` function with one argument (the calling Series or DataFrame) and
       that returns valid output for indexing (one of the above).
@@ -335,8 +335,7 @@ The ``.loc`` attribute is the primary access method. The following are valid inp
 * 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, when present in the
-  index! See :ref:`Slicing with labels
-  <indexing.slicing_with_labels>`.).
+  index! See :ref:`Slicing with labels <indexing.slicing_with_labels>`.
 * A boolean array.
 * A ``callable``, see :ref:`Selection By Callable <indexing.callable>`.
 
@@ -418,6 +417,9 @@ error will be raised (since doing otherwise would be computationally expensive,
 as well as potentially ambiguous for mixed type indexes). For instance, in the
 above example, ``s.loc[1:6]`` would raise ``KeyError``.
 
+For the rationale behind this behavior, see
+:ref:`Endpoints are inclusive <advanced.endpoints_are_inclusive>`.
+
 .. _indexing.integer:
 
 Selection by position