Fix CheckedShl/CheckedShr documentation
Fix CheckedShl/CheckedShr documentation

Fix #57 and more:

- CheckedShl was hinting that None was returned on overflow rather than
  on too large a rhs.
- Ditto for CheckedShr.
- CheckedShr documentation erroneously indicated that a left shift was
  going to be performed instead of a right shift.

rust-num/num-traits,rust-num/num-traits