Fix warnings and code styling in the doc

There was a blank line missing that was generating a warning when
building the doc. Also, a removed function from cli.py was left in api.rst
(without_appcontext specifically) so this was also generating warnings.

Also corrects the format for a code block and adds a link to Werkzeug's Headers
class.
Merge pull request #1161 from pbu88/master

Fix warnings and code styling in the doc