This updates footnote descriptions for 04005 & 04018.

Solves #42044547 in Pivotal.
Use https://www.gov.uk/trade-tariff/commodities/0808108090#footnotes for
verification after deploy.
Merge pull request #44 from alphagov/update_footnotes_04005_04018

This updates footnote descriptions for 04005 & 04018.