Explain why to focus on the end-user's need, and/or moderate that advice

[jyasskin]

In #39 (comment), in response to

Clearly articulate the problem that your proposed feature aims to solve from the user's perspective.

@chrishtr suggested

I think this needs more detail, and an example to show how it works. It also needs some sentences that explain why it's so important to start with the user-facing problem and not immediately get into the technical details.

We should also think through and explain how we want explainers to start from user needs when it's really a feature being designed for authors. E.g. CSS custom functions and signature-based integrity are primarily aimed at the second level of the Priority of Constituencies. While we need to ensure users aren't hurt, and there is some benefit to users from both of these, it's not inherently bad to add a feature for authors.