update wording around pep8 exceptions

At least once a cycle we get contributions from new folks that attempt
to update a bunch of pep8 formatting. They believe that this is in
good faith helping, but don't realize we really aren't likely to
accept that.

Update the text here to explain that more clearly, so that people
don't keep falling down this rabbit hole.

Change-Id: Iec383a18cb3d64b9655e5bd1d82cabd3e785686c

tox.ini

@@ -106,9 +106,18 @@ install_command = pip install -U --force-reinstall {opts} {packages}
 commands = bash -c tools/releasenotes_tox.sh
 
 [flake8]
-# E125 is deliberately excluded. See https://github.com/jcrocholl/pep8/issues/126
-# The rest of the ignores are TODOs
-# New from hacking 0.9: E129, E131, H407, H405
+# E125 is deliberately excluded. See
+# https://github.com/jcrocholl/pep8/issues/126. It's just wrong.
+#
+# Most of the whitespace related rules (E12* and E131) are excluded
+# because while they are often useful guidelines, strict adherence to
+# them ends up causing some really odd code formatting and forced
+# extra line breaks. Updating code to enforce these will be a hard sell.
+#
+# H405 is another one that is good as a guideline, but sometimes
+# multiline doc strings just don't have a natural summary
+# line. Rejecting code for this reason is wrong.
+#
 # E251 Skipped due to https://github.com/jcrocholl/pep8/issues/301
 
 ignore = E121,E122,E123,E124,E125,E126,E127,E128,E129,E131,E251,H405