docs(source-map): updated API section based on #207

[alamhubb]

Hello, I'm trying to develop a custom syntax lsp plugin based on ts recently, using volar.js, which is really great, saving me a lot of work and letting me learn a lot.

I'm learning about the functions of the code mapping related parts, and I found that the document description in

https://github.com/volarjs/volar.js/blob/master/packages/source-map/README.md

and the code implementation in

https://github.com/volarjs/volar.js/blob/master/packages/source-map/lib/sourceMap.ts are inconsistent, so I tried to synchronize this document

[johnsoncodehk]

LGTM, thanks!

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