libstfinv [WP][DOC]: add a note regaring purpose of Fourier domain base class

src/libs/libstfinv/stfinvfourier.h

@@ -52,6 +52,12 @@ namespace stfinv {
    * This is just a base class.
    * The constructor is protected and should only be called from a derived
    * class.
+   * The intention of this class is to provide all processing steps common to
+   * all engines operating in the Fourier domain in one single base class.
+   * The individual engines of different Fourier domain approaches then need
+   * not reimplement these steps.
+   * They essentially need only provide a specific exec-function (e.g.
+   * STFEngineFDLeastSquares::exec).
    *
    * This class maintains a workspace for Fourier transforms.
    * It provides the FFT from input signals to the workspace through a member