Clarify argument type

src/vs/editor/common/model.ts

@@ -1088,7 +1088,7 @@ export interface ITextBuffer {
 	getLineFirstNonWhitespaceColumn(lineNumber: number): number;
 	getLineLastNonWhitespaceColumn(lineNumber: number): number;
 
-	setEOL(newEOL: string): void;
+	setEOL(newEOL: '\r\n' | '\n'): void;
 	applyEdits(rawOperations: IIdentifiedSingleEditOperation[], recordTrimAutoWhitespace: boolean): ApplyEditsResult;
 }
 

src/vs/editor/common/model/linesTextBuffer/linesTextBuffer.ts

@@ -232,7 +232,7 @@ export class LinesTextBuffer implements ITextBuffer {
 
 	//#region Editing
 
-	public setEOL(newEOL: string): void {
+	public setEOL(newEOL: '\r\n' | '\n'): void {
 		this._EOL = newEOL;
 		this._constructLineStarts();
 	}