Mention that std::fs::remove_dir_all fails on files

This is explicitly mentioned for std::fs::remove_file's documentation,
but not in the aforementioned function.

It is more likely for a slightly lazy programmer to believe that
removing a file would work and that they do not have to distinguish
between directories (with contents) and files themself, because of the
function's recursive nature and how it distinguishes between files and
directories when removing them.

std/src/fs.rs

@@ -2804,8 +2804,9 @@ pub fn remove_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
 ///
 /// See [`fs::remove_file`] and [`fs::remove_dir`].
 ///
-/// `remove_dir_all` will fail if `remove_dir` or `remove_file` fail on any constituent paths, including the root path.
+/// `remove_dir_all` will fail if `remove_dir` or `remove_file` fail on any constituent paths, including the root `path`.
 /// As a result, the directory you are deleting must exist, meaning that this function is not idempotent.
+/// Additionally, `remove_dir_all` will also fail if the `path` is not a directory.
 ///
 /// Consider ignoring the error if validating the removal is not required for your use case.
 ///