Clarify comments

Author: Yui T

src/services/services.ts

@@ -1667,7 +1667,7 @@ module ts {
             var allDiagnostics = checker.getDiagnostics(targetSourceFile);
             if (compilerOptions.declaration) {
                 // If '-d' is enabled, check for emitter error. One example of emitter error is export class implements non-export interface
-                // Get emitter-diagnostics requires calling TypeChecker.emitFiles so define CompilerHost.writer which does nothing as this is a side effect of emitFiles
+                // Get emitter-diagnostics requires calling TypeChecker.emitFiles so we have to define CompilerHost.writer which does nothing because emitFiles function has side effects defined by CompilerHost.writer
                 var savedWriter = writer;
                 writer = (filename: string, data: string, writeByteOrderMark: boolean) => { };
                 allDiagnostics = allDiagnostics.concat(checker.emitFiles(targetSourceFile).errors);