jsc.baseUrl docs are confusing

[PeterJCLaw]

The docs for the jsc.baseUrl setting are incredibly minimal -- merely a link to some typescript docs. Unfortunately Typescript's docs have changed since the link was created, and no longer offer an immediately useful explanation for what value should be used. The page in question no longer seems to exist, with users instead redirected to general background reading on module resolution.

Ideally the docs would be reasonable self-sufficient, at least providing a basic summary of what the config option does, even if they link out to other sources for background context.

Moved from swc-project/swc#8605.