tidying comments/header

src/new/thermal/FT_temperature_field.m

@@ -5,6 +5,8 @@
 % to an incidnet beam and absorption in the bulk (substrate) or coating of
 % the mirror.
 %
+% N.B. currently only valid for incident LG00 mode.
+%
 % beam:     Structure containing beam parameters (FT_init_thermal_beam.m)
 % mirror:   Structure containing mirror parameters 
 %           (FT_init_thermal_mirror.m)