Reword to emphasize borrow isn't permanent

I thought it's wording suggested that after a single mutable borrow, an immutable borrow will never be allowed again.  I tried to reword it to emphasize that the borrow is only disallowed _while_ the borrow is still active.

The end sentence was inverted (can be vs. can't be) to emphasize that another borrow _will_ be allowed but will require a condition.