Edits to trees.py following Peter's comments

Author: apragsdale

python/tskit/trees.py

@@ -10966,11 +10966,13 @@ def ld_matrix(
         in a square matrix (see :ref:`sec_stats_two_locus_branch` for
         examples).
 
-        Some LD statistics are defined for two sample sets instead of within a
-        single set of samples. If the ``indexes`` argument is specified, at
-        least two sample sets must also be specified. ``indexes`` specifies the
-        indexes of the sample sets in the ``sample_sets`` list
-        between which to compute LD.
+        Some LD statistics are defined for two sample sets as well as within a
+        single set of samples. If the ``indexes`` argument is specified, then
+        ``indexes`` specifies the indexes of the sample sets in the
+        ``sample_sets`` list between which to compute LD. For example, this
+        results in a 3D array whose ``[k,:,:]``-th slice contains LD values
+        between ``sample_sets[i]`` and ``sample_sets[j]``, where ``(i, j)`` is
+        the ``k``-th element of ``indexes``.
 
         For more on how the ``indexes`` and ``sample_sets`` interact with the
         output dimensions, see the :ref:`sec_stats_two_locus_sample_sets`
@@ -10979,7 +10981,7 @@ def ld_matrix(
         **Available Stats** (use ``Stat Name`` in the ``stat`` keyword
         argument). Statistics marked as "multi sample set" allow
         (but do not require) computation from two sample sets
-        via the ``indexes`` argument. 
+        via the ``indexes`` argument.
 
         ======================= ========== ================ ==============
         Stat                     Polarised Multi Sample Set Stat Name
@@ -10996,9 +10998,9 @@ def ld_matrix(
         :math:`\widehat{\pi_2}`  n          n               "pi2_unbiased"
         ======================= ========== ================ ==============
 
-        :param list sample_sets: A list, or a list of lists of sample node IDs, specifying
-            the groups of nodes to compute the statistic with. Defaults to all
-            samples grouped by population.
+        :param list sample_sets: A list, or a list of lists of sample node IDs,
+            specifying the groups of nodes to compute the statistic with. Defaults
+            to all samples.
         :param str mode: A string giving the "type" of the statistic to be
             computed. Defaults to "site", can be "site" or "branch".
         :param str stat: A string giving the selected two-locus statistic to