Doc fixes for core::future::Future.

Fixed outdated reference to `waker` argument; now futures are passed a
`Context`, from which one can obtain a `waker`.

Cleaned up explanation of what happens when you call `poll` on a completed
future. It doesn't make sense to say that `poll` implementations can't cause
memory unsafety; no safe function is ever allowed to cause memory unsafety, so
why mention it here? It seems like the intent is to say that the `Future` trait
doesn't say what the consequences of excess polls will be, and they might be
bad; but that the usual constraints that Rust imposes on any non-`unsafe`
function still apply. It's also oddly specific to say 'memory corruption'
instead of just 'undefined behavior'; UB is a bit jargony, so the text should
provide examples.

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