TransientPropagation: fix documentation
This MR fixes a few documentation issues found in the TransientPropagation:
- Sectioning was nor properly done, resulting in a wring TOC in the manual
- The README was not up-to-date with the code of the module, also see https://allpix-squared-forum.web.cern.ch/t/using-pulsetransfer-module/98
Edited by Simon Spannagel