Ensure that doc linting is included in the linters test
The 'docs' tox target executes the doc8 lint test which may result in
failures when testing documentation builds, but OpenStack-CI does not
execute that tox target.
In order to ensure that we catch all standard documentation syntax
errors and prevent them from merging, this patch includes the docs
target in the 'linters' chain of tests.
Fixes for any failures which result from executing this test are also
included in the patch.