Update the Topics API description

[captainbrosset]

I think this could just be: "The Topics API enables interest-based advertising without using third-party cookies."

If you need more: "The Topics API can be used to observe and provide access to topics that appear to be of interest to the user, based on their activity. The Topics API can then give API callers (such as ad tech platforms) access to a user's topics of interest, but without revealing additional information about the user's activity."

Originally posted by @samdutton in #3526 (comment)