Consider reworking "background" section of spec

[jkup]

I'm a little unsure what to glean from this section https://tc39.es/source-map/#background

It isn't immediately clear to me what changes were made in v2 and v3 to change the source map size. I'm also bit concerned with the entire background paragraph saying "we keep making source maps smaller" when, in reality, we're considering adding a lot more information to them (scopes, module graph information, custom formatters). I think it would be cool if we had a different Background section.