DiffService: move the docstring to package level

As more comparison tests are going to be added, it's better to have
this docstring (clarifying the motive and explaining few things about
these tests) at package level.
2 jobs for !32 with topic/default/rawDiff in 32 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Compat
failed #167899
allowed to fail
tests-hg-default

00:00:22

passed #167898
tests-hg-stable

00:00:32

100.0%
 
Name Stage Failure
failed
tests-hg-default Compat
    writing top-level names to pip-egg-info/mercurial.egg-info/top_level.txt
writing manifest file 'pip-egg-info/mercurial.egg-info/SOURCES.txt'
Python headers are required to build Mercurial but weren't found in /usr/include/python3.7m/Python.h

----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-0mfsuu95/Mercurial/
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1