Update documentation on retiring
Update documentation on retiring

This commit updates the retire a smart answer document, highlighting
the recent increase in the number of content items that make up a smart
answer.

aledelcueto/smart-answers,alphagov/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers