fix the std::fs::create_dir_all docs

Author: fluiderson

library/std/src/fs.rs

@@ -2803,6 +2803,9 @@ pub fn create_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// Recursively create a directory and all of its parent components if they
 /// are missing.
 ///
+/// This function returns an error if any of the parent components are not a directory and already
+/// exist.
+///
 /// If the empty path is passed to this function, it always succeeds without
 /// creating any directories.
 ///