Structure alternatives as: Alternative → Pros → Cons → Reason for rejection.

[jyasskin]

Thanks to @toreini for the suggestion. Otherwise we get sections like https://github.com/explainers-by-googlers/script-src-v2/blob/main/README.md#considered-alternatives that mix the definition of the alternative with its disadvantages, and often don't mention advantages at all.

We should consider whether Pros/Cons or Advantages/Disadvantages is more in line with https://github.com/w3ctag/process/blob/main/style-guide.md.

[jyasskin]

Another option for restructuring the alternatives: #35

I believe @matatk said he was going to look into making the patch to the explainer explainer. I think it's fine to do this smaller change before figuring out #35.