pattern_sim: Update help

src/pattern_sim.c

@@ -62,15 +62,21 @@ static void show_help(const char *s)
 "     --powder              Write a summed pattern of all images simulated by\n"
 "                            this invocation to results/integr.h5.\n"
 " -i, --intensities=<file>  Specify file containing reflection intensities\n"
-"                            to use.\n"
+"                            (and phases) to use.\n"
 " -g, --gradients=<method>  Use <method> for the calculation of shape\n"
 "                            transform intensities.  Choose from:\n"
 "                             mosaic      : Take the intensity of the nearest\n"
-"                                           Bragg position.\n"
+"                                           Bragg position.  This is the\n"
+"                                           fastest method and the only one\n"
+"                                           supported on the GPU.\n"
 "                             interpolate : Interpolate trilinearly between\n"
 "                                           six adjacent Bragg intensities.\n"
+"                                           This method has intermediate\n"
+"                                           accuracy.\n"
 "                             phased      : As 'interpolate', but take phase\n"
-"                                           values into account.\n"
+"                                           values into account.  This is the\n"
+"                                           most accurate method, but the\n"
+"                                           slowest.\n"
 "\n"
 "By default, the simulation aims to be as accurate as possible.  For greater\n"
 "speed, or for testing, you can choose to disable certain things using the\n"