doc: clarify documentation of potentials

local_r4: make clear that the potential is shifted before the smoothing
          function is multiplied

          use 'g' rather than 'f' for the function to avoid potential
          confusion with the force.

lennard_jones_linear: fix typo
io/writers/h5md: allow for non-decreasing time dataset

This relaxed condition corresponds to H5MD 1.0.1 and allows us to change
the state of the simulation without incrementing time, e.g., rescaling
of the velocities.
avoid exception in H5MD output of trajectories

If the sampling interval is omitted in the phase space writer, a dump of
the system state is triggered by the signals "start"and "finish" of
observables.sampler. These two signals are always emitted by
sampler:run(). Repeated calls lead to duplicate dumps at the _same_
simulation step, which is not valid H5MD.

The following workarounds are possible:

1) remove the signals "start", "finish" from sampler:run()

2) increment the step counter (but not the clock) after each call to
   sampler:run()

3) avoid connection of the phase space writer to start/finish by
   explicitly passing a sampling interval, e.g., the total number of
   simulation steps.

This patch applies the last option and fixes #286246.
doc: fix module path in example