Improve style guide to clarify common issues. (#2857)
Improve style guide to clarify common issues. (#2857)

* Improve style guide to clarify common issues.

These changes are meant to clarify some common questions I've encountered during
content review. I've also removed the exception for passive voice since there is
no need to obscure the subject of a sentence in documentation. I've also
introduced the concepts of title case and sentence case to align with other
style guides for clarity.

geeknoid/istio.github.io,istio/istio.io,istio/istio.io,istio/istio.io,istio/istio.io,geeknoid/istio.github.io,geeknoid/istio.github.io,geeknoid/istio.github.io,istio/istio.io,geeknoid/istio.github.io,istio/istio.io,istio/istio.io