List filter identifiers in mcdoc string

Author: willend

mcstas-comps/examples/Tests_optics/Test_filters/Test_filters.instr

@@ -23,7 +23,7 @@
 * Lmin:         [Angs] Lowest wavelength from source
 * Lmax:         [Angs] Highest wavelength from source
 * nL:              [1] Number of wavelength bins in [Lmin Lmax] interval
-* Filter:          [1] Choice of filter: Filter_gen, NCrystal, PowderN, Isotropic_Sqw
+* Filter:          [1] Choice of filter 0: Filter_gen, 1: NCrystal, 2: PowderN, 3: Isotropic_Sqw
 * zdepth:          [m] Depth of Be-filter
 * T:               [K] Filter temperature (NCrystal only)
 * %Link