Explaining the reason why validation is performed in to_str of path.rs

src/libstd/path.rs

@@ -1819,6 +1819,8 @@ impl Path {
     /// Yields a [`&str`] slice if the `Path` is valid unicode.
     ///
     /// This conversion may entail doing a check for UTF-8 validity.
+    /// Also it it worthwhile noting that validation is performed because Non-UTF-8 strings are
+    /// perfectly valid for some OS.
     ///
     /// [`&str`]: ../primitive.str.html
     ///