Fixed comment of as_mut_slice (libstd/option.rs)

The old comment did not describe the function correctly
Refactored iter and mut_iter

Replaced match by self.as_ref() and self.as_mut()
auto merge of #12888 : aochagavia/rust/Fix-comment, r=alexcrichton

The old comment of as_mut_slice() did not describe the function correctly. The new one does.

Also refactored option::iter() and option::mut_iter() to use as_ref() and as_mut() instead of match.