add guarantee about remove_dir and remove_file error kinds

approved in ACP rust-lang/libs-team#433

Author: lolbinarycat

library/std/src/fs.rs

@@ -1989,6 +1989,11 @@ impl AsInner<fs_imp::DirEntry> for DirEntry {
 /// * The file doesn't exist.
 /// * The user lacks permissions to remove the file.
 ///
+/// This function will only ever return an error of kind `NotFound` if the given
+/// path does not exist.  Note that the inverse is not true,
+/// ie. if a path does not exist, its removal may fail for a number of reasons,
+/// such as insufficient permissions.
+///
 /// # Examples
 ///
 /// ```no_run
@@ -2446,6 +2451,11 @@ pub fn create_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// * The user lacks permissions to remove the directory at the provided `path`.
 /// * The directory isn't empty.
 ///
+/// This function will only ever return an error of kind `NotFound` if the given
+/// path does not exist.  Note that the inverse is not true,
+/// ie. if a path does not exist, its removal may fail for a number of reasons,
+/// such as insufficient permissions.
+///
 /// # Examples
 ///
 /// ```no_run