[TASK][DOC] (libstfinv): specify meaning of offset dependent weight

The weight factors are applied in terms of signal amplitude not in terms of
signal energy. Where factors are to be applied to energy values, they have to
be squared prior to the operation.

src/libs/libstfinv/usage/stfinvbase_description_usage.txt

@@ -12,5 +12,7 @@ larger offset would contribute less to the optimization criterion for which
 the source wavelet correction filter is constructed. The option exp provides
 means to add a weight factor ((r/1m)**k) to each signal, where r is the
 receiver to source offset. This is used to compensate the decrease in signal
-energy.
+amplitude. If the energy in the original signal decays with ((-r/1m)**(2*k))
+all traces will contribute at a similar level to the derived correction filter
+after application of the weight factors.
 # ----- END OF stfinvbase_description_usage.txt -----