Updated fix gcmc docs for tail correction note

NOTE: If the density of the cell is initially very small or zero, and

increases to a much larger density after a period of equilibration,

then certain quantities that are only calculated once at the start

(kspace parameters, tail corrections) may no longer be accurate.  The

(kspace parameters) may no longer be accurate.  The

solution is to start a new simulation after the equilibrium density

has been reached.