Documentation fix: Rerun and nstlist

Documentation fix: Rerun and nstlist

The current code is not, as written in the documentation, behaving differently if
`nstlist == 0`. The relevant code part in `md.cpp` (line 312+) reads

    const gmx_bool bRerunMD      = mdrunOptions.rerun;
    int            nstglobalcomm = mdrunOptions.globalCommunicationInterval;

    if (bRerunMD)
    {
        /* Since we don't know if the frames read are related in any way,
         * rebuild the neighborlist at every step.
         */
        ir->nstlist       = 1;
        ir->nstcalcenergy = 1;
        nstglobalcomm     = 1;
    }

This change adapts the documentation to reflect what we're is actually doing.

Change-Id: I2292c675e0798f7e500ac3438a61872accabf21c