Try to make it clearer how these are different from Attributes. Resolves #1776.

Also adds a docstring to the is_discountable AbstractProduct

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

Fixes #697

* Better wordings for the individual steps
* Mentioned necessary copying of migrations directory
* Fixed wrong prerequisites in examples
* Glossary typo fixed

Commonly used terms should be explained there.