As it came up on the mailing list again, I've clarified a few more
scenarios of what needs to be done with migrations when upgrading.

Those documents lived in ref/, but really aren't reference documents.
Topics/ is a more appropriate place for them.

* Use correct directives in signals.rst
* Removed newline in glossary.rst that broke sorting of glossary terms
* Removed syntax warning for upgrading.rst

Fixes #608