update language on contact index page

![image](https://f.cloud.github.com/assets/23801/1466284/e224c220-456e-11e3-8ded-fef7c5fe37e5.png)

this change:

* simplifies the first sentence
* moves the link to the help pages to the bottom, because data shows that few people
click on
* brings the description of the GOV.UK contact form closer in line with the User Support
team's actual remit