doc: add comment

Author: CGNonofr

src/types/monaco.d.ts

@@ -3,6 +3,8 @@ import { IRange } from 'monaco-editor'
 declare module 'monaco-editor' {
     namespace editor {
         interface IStandaloneCodeEditor {
+            // This method is internal and is supposed to be used by the folding feature
+            // We still use it to hide parts of the code in the `hideCodeWithoutDecoration` function
             setHiddenAreas(ranges: IRange[]): void
         }
     }