Document current default permissions for tempdirs (#296)

Fixes implying that the default is the same as the one for tempfiles

Author: Imberflur

src/lib.rs

@@ -337,7 +337,6 @@ impl<'a, 'b> Builder<'a, 'b> {
  }
 
  /// The permissions to create the tempfile or [tempdir](Self::tempdir) with.
- /// This allows to them differ from the default mode of `0o600` on Unix.
  ///
  /// # Security
  ///
@@ -350,9 +349,13 @@ impl<'a, 'b> Builder<'a, 'b> {
  /// # Platform Notes
  /// ## Unix
  ///
- /// The actual permission bits set on the tempfile or tempdir will be affected by the
- /// `umask` applied by the underlying syscall.
+ /// The actual permission bits set on the tempfile or tempdir will be affected by the `umask`
+ /// applied by the underlying syscall. The actual permission bits are calculated via
+ /// `permissions & !umask`.
  ///
+ /// Permissions default to `0o600` for tempfiles and `0o777` for tempdirs. Note, this doesn't
+ /// include effects of the current `umask`. For example, combined with the standard umask
+ /// `0o022`, the defaults yield `0o600` for tempfiles and `0o755` for tempdirs.
  ///
  /// ## Windows and others
  ///