Updated @volar/source-map/README based on PR(#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

There is also a about

volar.js/packages/source-map/lib/sourceMap.ts

Line 71 in 03d1e8b

* findMatchingStartEnd(

* findMatchingStartEnd I am a little confused about this method. I see that in this method, when fallbackToAnyMatch = true is judged, it will break if it matches one, but the non-fallbackToAnyMatch code block above does not break. What is the reason?
I am also curious about the situation where there will be a start and end position of the source code, corresponding to the start and end positions of multiple generated codes.

[johnsoncodehk]

@alamhubb Sorry for the delay, would you like to reopen this PR? never mind, I see #248. :)

* findMatchingStartEnd I am a little confused about this method. I see that in this method, when fallbackToAnyMatch = true is judged, it will break if it matches one, but the non-fallbackToAnyMatch code block above does not break. What is the reason?

The mapping between virtual code and source code may be discrete or continuous. Some functions should only be mapped in a continuous range (such as formatting). fallbackToAnyMatch is used to control whether to try to map start/end to a discrete mapping range when a continuous mapping range cannot be successfully mapped.