doc nit: no longer so many warnings for implicit-fallthrough

doc nit: no longer so many warnings for implicit-fallthrough

The old pcre code included with Privoxy had lots of fallthroughs
but that code is gone and the only implicit-fallthrough left is:

w32log.c: In function ‘LogWindowProc’:
w32log.c:1189:27: warning: this statement may fall through [-Wimplicit-fallthrough=]
 1189 |          g_bShowLogWindow = wParam;
      |          ~~~~~~~~~~~~~~~~~^~~~~~~~
w32log.c:1190:7: note: here
 1190 |       case WM_SIZE:  /* note: implicit-fallthrough */
      |       ^~~~