Tweak the descriptions for topics in search

The previous wording was "Everything on GOV.UK about ..." which was
actually incorrect.  This commit changes it to "List of information
about ..." which is correct, but not great.

Future work will seek to improve this.
Merge pull request #705 from alphagov/tweak-wording-for-subtopics-in-search

Tweak the descriptions for topics in search