Add doc comment

src/libsyntax/parse/mod.rs

@@ -330,6 +330,16 @@ pub fn stream_to_parser(sess: &ParseSess, stream: TokenStream) -> Parser<'_> {
 }
 
 /// Given stream, the `ParseSess` and the base directory, produces a parser.
+///
+/// Use this function when you are creating a parser from the token stream
+/// and also care about the current working directory of the parser (e.g.,
+/// you are trying to resolve modules defined inside a macro invocation).
+/// 
+/// # Note
+///
+/// The main usage of this function is outside of rustc, for those who uses
+/// libsyntax as a library. Please do not remove this function while refactoring
+/// just because it is not used in rustc codebase!
 pub fn stream_to_parser_with_base_dir<'a>(sess: &'a ParseSess,
                                           stream: TokenStream,
                                           base_dir: Directory<'a>) -> Parser<'a> {