STYLE: HashTable documentation

- explicitly mention the value-initialized status for the operator().
  This means that the following code will properly use an initialized
  zero.

      HashTable<label> regionCount;

      if (...)
         regionCount("region1")++;

      ... and also this;

      if (regionCount("something") > 0)
      {
          ...
      }

  Note that the OpenFOAM HashTable uses operator[] to provide read and
  write access to *existing* entries and will provoke a FatalError if
  the entry does not exist.

  The operator() provides write access to *existing* entries or will
  create the new entry as required.
  The STL hashes use operator[] for this purpose.