reproducible Debian: correctly document that we are not variying usrmerge (see...

reproducible Debian: correctly document that we are not variying usrmerge (see below for explaination)

because:

- Debian is still not using usrmerge on buildds
- this is a noop-change for r-b
- I'd like the jenkins hosts not to reach unworking load levels. This seems to
  diffoscope related/caused, so I'm trying to reduce variations to get
  diffoscope not to act up and kill the host almost daily

Signed-off-by: Holger Levsen <holger@layer-acht.org>