Basic SEO question I can't find the answer to

Let's say I am writing an article titled "The Best Shampoo for your Dog". To optimize for SEO, should I only write about the best shampoos for your dog? Is it harmful to also have a section on best practices for washing your dog after the best shampoo was discussed?

I guess my overarching question is how laser-focused on the topic should I be vs also including auxiliary information that is adjacent to the topic?