Add a docstring

src/nexusformat/nexus/tree.py

@@ -5233,6 +5233,7 @@ def __setitem__(self, idx, value):
             self.nxlink.__setitem__(idx, value)
 
     def __eq__(self, other):
+        """Return True if two linked objects share the same target."""
         if isinstance(other, NXlink):
             return ((self._target == other._target) and
                     (self._filename == other._filename))