The customisation section required some changes as it's not often
necessary to override the root application instance to change URLs or
views of subapps.

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

The mailing list raised the issue of having to include the models.py to
replace a Form. The docs weren't clear, so I've taken the opportunity
to:

* merge 'INSTALLED_APPS' section and 'models.py & admin.py'.
* change misc wordings and add clarifications

Attempt at making it clearer what needs to be done for what kind of
customisation. Created a new topic that lines out the prepatory
steps that need to be taken, and moved duplicate information from the
how-tos into the new topic.