Add a link to the subset docs from union

Author: hyanwong

docs/python-api.md

@@ -262,10 +262,12 @@ which perform the same actions but modify the {class}`TableCollection` in place.
   TreeSequence.simplify
   TreeSequence.subset
   TreeSequence.union
+  TreeSequence.concatenate
   TreeSequence.keep_intervals
   TreeSequence.delete_intervals
   TreeSequence.delete_sites
   TreeSequence.trim
+  TreeSequence.shift
   TreeSequence.split_edges
   TreeSequence.decapitate
   TreeSequence.extend_haplotypes
@@ -750,7 +752,9 @@ a functional way, returning a new tree sequence while leaving the original uncha
   TableCollection.keep_intervals
   TableCollection.delete_sites
   TableCollection.trim
+  TableCollection.shift
   TableCollection.union
+  TableCollection.concatenate
   TableCollection.delete_older
 ```
 

python/tskit/trees.py

@@ -7068,6 +7068,10 @@ def shift(self, value, sequence_length=None, record_provenance=True):
         ``value``, unless ``sequence_length`` is given, in which case this will be used
         for the new sequence length.
 
+        .. note::
+            By setting ``value=0``, this method will simply return a tree sequence
+            with a new sequence length.
+
         :param value: The amount by which to shift the coordinate system.
         :param sequence_length: The new sequence length of the tree sequence. If
             ``None`` (default) add ``value`` to the sequence length.
@@ -7366,9 +7370,9 @@ def union(
             ``other`` to nodes in ``self``.
         :param bool check_shared_equality: If True, the shared portions of the
             tree sequences will be checked for equality. It does so by
-            subsetting both ``self`` and ``other`` on the equivalent nodes
-            specified in ``node_mapping``, and then checking for equality of
-            the subsets.
+            running :meth:`TreeSequence.subset` on both ``self`` and ``other``
+            for the equivalent nodes specified in ``node_mapping``, and then
+            checking for equality of the subsets.
         :param bool add_populations: If True, nodes new to ``self`` will be
             assigned new population IDs.
         :param bool record_provenance: Whether to record a provenance entry