Error on warnings in building the API guide and fix any problem
encountered:
* tox.ini: Enable tread warnings as errors
* index.rst: Add unreferenced file extra_specs_and_properties as
hidden file so that it does not show up in index. It is still
built and will be referenced from other files.
* general_info.rst: Remove reference to non-existing file
* server_concept: Add missing empty line after code directives
Also fix a typo and remove extra EOL whitespace.
Change-Id: Ie766f8d2f7d82b7b5b02541b342f9829ca4550a0