I struggled with migrations when upgrading from Oscar 2.0.4 to 2.1.
@solarissmoke kindly helped out, and I want to say thank you by adding
the bits I learned to the docs.

I also updated the handling migration section of the general upgrading
doc to use native migrations lingo instead of South's.

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