libstfiv [WP][DOC]: add note regarding implicit periodicity

src/libs/libstfinv/usage/stfinvfourier_description_usage.txt

@@ -41,10 +41,11 @@ A time domain taper can be applied to the impulse response of the correction
 filter by using option irtap. Four time values are given in units of seconds:
 t1, t2, t3, and t4. They must be in increasing order and (t4-t1) must be
 smaller than the total duration of the time series used to represent signals
-internally. Time value are allowed to be negative. Prior to application of the
-correction filter to the time series passed to the algorithm, the correction
-filter is transformed to the time domain, tapered, and then transformed to the
-Fourier domain again. The values of the taper are:
+internally. Times value are allowed to be negative. Time series are understood
+to be periodic (due to discrete Fourier transformation). Prior to application
+of the correction filter to the time series passed to the algorithm, the
+correction filter is transformed to the time domain, tapered, and then
+transformed to the Fourier domain again. The values of the taper are:
   0                              if       t <  t1
   0.5-0.5*cos(pi*(t-t1)/(t2-t1)) if t1 <= t <= t2
   1                              if t2 <  t <  t3