Cleanup: Add docstring to StrokeCache bstrength

Pull Request: https://projects.blender.org/blender/blender/pulls/123143

source/blender/editors/sculpt_paint/sculpt_intern.hh

@@ -389,6 +389,10 @@ struct StrokeCache {
   bool pen_flip;
   bool invert;
   float pressure;
+  /**
+   * Depending on the mode, can either be the raw brush strength, or a scaled (possibly negative)
+   * value. See #brush_strength for Sculpt Mode.
+   */
   float bstrength;
   float normal_weight; /* from brush (with optional override) */
   float x_tilt;