Added NoJump on-the-fly example to MSD documentation (Issue #4169) (#5165)

* Fixes #4169
* This PR updates the MSD documentation inside MDAnalysis.analysis.msd to show users 
   how to correctly apply the NoJump transformation using on-the-fly trajectory transformations.
    - A minimal working example demonstrating NoJump usage
    - Clarification that wrapped coordinates must be unwrapped
    - A modern MDAnalysis-based workflow that does not require external tools
* The example was tested, with scripts and output available on the PR #5165

Author: tani-dubey

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.