Document rationale when conventions do not follow existing guidance

[trask]

Sometimes there's a good reason not to follow existing guidance, but it can be hard to review semantic conventions without knowing this reasoning, or understanding the scope that the reasoning applies to, e.g. https://github.com/open-telemetry/semantic-conventions/pull/2180#discussion_r2060698638.

It would be great if these rationales were documented somewhere under docs/non-normative.

We could even have one doc for the rationale "stabilized already 🤷".