Docs style fixes: ``FOO`` vs `FOO`

I chose the option that is used more often in documentation:

         `True`, `False`, `None` for constants
         `GET`, `POST`, `HEAD` for HTTP methods
         `app.py`, `requirements.txt` for file names
         `pip`, `easy_install` for executables
         `static`, `templates` for folder names

But:

        ``DEBUG``, ``TESTING`` for config options
        ``/``, ``/backend`` for URL paths
        ``/var/www/yourapplication`` for filesystem paths
        ``X-Forwarded-Host``, ``X-Forwarded-For`` for HTTP headers
        ``Flask-Uploads``, ``Flask-SqlAlchemy`` for extension names (at least at the first mention)
        ``template.html``, ``layout.html`` for Jinja templates
        ``<body>``, ``<script>`` for HTML tags
        ``|safe``, ``{% extends %}`` for Jinja filters and codeblocks

And:

        HTTP, not `HTTP`

And yet it seems that `requirements.rst` should be a `requirements.txt`.

During edits sense was not injured.