Stop documenting the extinct prepend option for project references (microsoft#3239)

Author: mkantor

packages/documentation/copy/en/project-config/Project References.md

@@ -92,34 +92,6 @@ Enabling the [`composite`](/tsconfig#composite) flag changes a few things:
 We've also added support for [declaration source maps](https://github.com/Microsoft/TypeScript/issues/14479).
 If you enable [`declarationMap`](/tsconfig#declarationMap), you'll be able to use editor features like "Go to Definition" and Rename to transparently navigate and edit code across project boundaries in supported editors.
 
-## `prepend` with `outFile`
-
-You can also enable prepending the output of a dependency using the `prepend` option in a reference:
-
-```js
-   "references": [
-       { "path": "../utils", "prepend": true }
-   ]
-```
-
-Prepending a project will include the project's output above the output of the current project.
-All output files (`.js`, `.d.ts`, `.js.map`, `.d.ts.map`) will be emitted correctly.
-
-`tsc` will only ever use existing files on disk to do this process, so it's possible to create a project where a correct output file can't be generated because some project's output would be present more than once in the resulting file.
-For example:
-
-```txt
-   A
-  ^ ^
- /   \
-B     C
- ^   ^
-  \ /
-   D
-```
-
-It's important in this situation to not prepend at each reference, because you'll end up with two copies of `A` in the output of `D` - this can lead to unexpected results.
-
 ## Caveats for Project References
 
 Project references have a few trade-offs you should be aware of.
@@ -211,7 +183,6 @@ You will need to either set the [`outDir`](/tsconfig#outDir) to an explicit subf
 ### Structuring for outFiles
 
 Layout for compilations using [`outFile`](/tsconfig#outFile) is more flexible because relative paths don't matter as much.
-One thing to keep in mind is that you'll generally want to not use `prepend` until the "last" project - this will improve build times and reduce the amount of I/O needed in any given build.
 The TypeScript repo itself is a good reference here - we have some "library" projects and some "endpoint" projects; "endpoint" projects are kept as small as possible and pull in only the libraries they need.
 
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