Document default tsBuildInfoFile defaults

I was going to report a bug because the actual default of tsbuildinfo
does not match the docs for --tsBuildInfoFile. I found [this issue] was
already reported and closed as 'working as intended'.

So I decided to add @sheetalkamat's response to the docs.

[this issue]: microsoft/TypeScript#30925

packages/tsconfig-reference/copy/en/options/tsBuildInfoFile.md

@@ -7,4 +7,17 @@ This setting lets you specify a file for storing incremental compilation informa
 building of larger TypeScript codebases. You can read more about composite projects [in the handbook](/docs/handbook/project-references.html).
 
 This option offers a way to configure the place where TypeScript keeps track of the files it stores on the disk to
-indicate a project's build state — by default, they are in the same folder as your emitted JavaScript.
+indicate a project's build state.
+
+The default depends on a combination of other settings
+
+ - If `outFile` the file is `<outFile>.tsbuildinfo`
+
+ - If `rootDir` and `outDir` then the file is `<outDir>/<relative path to config from rootDir>/<config name>.tsbuildinfo`
+   For example if `rootDir` is `src` and `outDir` is `dest` and  the config is
+   `./tsconfig.json` then tsBuildInfoFile defaults to `./tsconfig.tsbuildinfo`
+   because the relative path from `src/` to `./tsconfig.json` is `../`.
+
+ - If just `outDir` then `<outDir>/<config name>.tsbuildInfo`
+
+ - otherwise `<config name>/.tsbuildInfo`