Update content for “Tax you pay on your pension” article.

See: https://docs.google.com/a/digital.cabinet-office.gov.uk/document/d/11Qs0aW98-1Y62pc4pXCKikBHxawCRbcnHGsp9NyHhe8/edit
Renumber sections of “Tax you pay on your pension” article after dc69c464.
Update booking process content and tighten up what the product communicates.

@mcornford :lollipop:
Merge branch 'ggp-20141106-alpha' of github.com:guidance-guarantee-programme/prototypes into ggp-20141106-alpha