Added missing doc file

Author: MichaelGoodale

docs/source/syntax.rst

@@ -0,0 +1,25 @@
+Syntax tree utilities
+=====================
+
+These are the classes that are useful for manipulating or plotting parse trees directly.
+
+.. autoclass:: python_mg.syntax.ParseTree
+   :members:
+   :undoc-members:
+
+.. autoclass:: python_mg.syntax.Mover
+   :members:
+   :undoc-members:
+
+.. autoclass:: python_mg.syntax.Trace
+   :members:
+   :undoc-members:
+
+.. autoclass:: python_mg.syntax.MGNode
+   :members:
+   :undoc-members:
+
+.. autoclass:: python_mg.syntax.MGEdge
+   :members:
+   :undoc-members:
+