Update python/tskit/trees.py

Co-authored-by: Peter Ralph <petrel.harp@gmail.com>

Author: apragsdale

python/tskit/trees.py

@@ -10956,10 +10956,11 @@ def ld_matrix(
         :ref:`sec_stats_two_locus_site` for examples).
 
         Similarly, in the branch mode, the ``positions`` argument specifies
-        loci for which the expectation for the two-locus statistic is computed
-        over pairs of trees at those positions. LD stats are computed between
-        trees whose ``[start, end)`` contains the given position (such that
-        repeats of trees are possible). Similar to the site mode, a nested list
+        genomic coordinates at which the expectation for the two-locus statistic
+        is computed, given the local tree structure. This defaults to computing
+        the LD for each pair of distinct trees, which is equivalent to passing in
+        the leftmost coordinates of each tree's span (since intervals are closed on
+        the left and open on the right). Similar to the site mode, a nested list
         of row and column positions can be specified separately (resulting in a
         rectangular matrix) or a single list of a specified positions results
         in a square matrix (see :ref:`sec_stats_two_locus_branch` for