Added NoJump on-the-fly example to MSD documentation (Issue #4169)

package/MDAnalysis/analysis/msd.py

@@ -70,7 +70,34 @@
 
    In MDAnalysis you can use the 
    :class:`~MDAnalysis.transformations.nojump.NoJump`
-   transformation. 
+   transformation to unwrap coordinates on-the-fly.
+
+    A minimal example:
+
+    .. code-block:: python
+
+        import MDAnalysis as mda
+        from MDAnalysis.transformations import NoJump
+
+        u = mda.Universe(TOP, TRAJ)
+
+        # Apply NoJump transformation to unwrap coordinates
+        u.trajectory.add_transformations(NoJump(u))
+
+        # Now the trajectory is unwrapped and MSD can be computed normally:
+        from MDAnalysis.analysis.msd import EinsteinMSD
+        MSD = EinsteinMSD(u, select="all", msd_type="xyz")
+        MSD.run()
+
+   This example assumes that the trajectory contains periodic box
+   dimensions. If no periodic boundary information is present, box
+   dimensions must be defined before applying ``NoJump``, which can
+   be accomplished by applying the
+   :class:`~MDAnalysis.transformations.boxdimensions.set_dimensions`
+   transformation *before* the 
+   :class:`~MDAnalysis.transformations.nojump.NoJump` transformation.
+
+   This replaces the need to preprocess trajectories externally.
 
    In GROMACS, for example, this can be done using `gmx trjconv`_ with the
    ``-pbc nojump`` flag.