Various documentation improvements

Here's a pile of documentation improvements. Some of them document new options added to git master today, but there's also clarifications and I added some docs I found were missing.

In some of these commits, I state that options introduced today in git master will be released in version 1.9.18, is that correct?

Commit "Document the set-placeholder and set-ph options" is written assuming my last comment at https://github.com/unbit/uwsgi/issues/405 will be resolved as suggested.