Reword the documentation regarding coding style

- flake8 is a wrapper around pyflakes, so it's redundant to mention
  both of them. Roll the explicit sections about coding errors and
  coding style violations into a single section.

- After recent cleanups the project now has zero warnings or errors
  from flake8. Reword the requirements so that it is now mandatory
  to not introduce new warnings.

- Expand the section on suppression of warnings to differentiate
  between suppressing inline individually and globally suppressing
  for the whole project.

- Properly capitalize "Python Style Guide".

Change-Id: I4b333d013e985db252873441b16cb719ed5be5b5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255040
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>