epoch: Fix defer_destroy doc example about not requiring T: Send

[teddytennant]

What

Replaces the doc example in Guard::defer_destroy that motivates the lack of a T: Send bound, and turns it from an ignore block into a compiling doctest.

Fixes #1253.

Why

The old snippet was copied from defer_unchecked and doesn't actually demonstrate the point: it defers destruction of an i32, which is Send, so it would compile fine even if defer_destroy required T: Send. The comment "Shared is not Send!" is beside the point, since the bound in question is on T, not on Shared<'_, T>.

The new example shows the actual typical case where the type system can't prove T: Send: a concurrent data structure node containing a raw pointer to another node. Raw pointers are not Send, so such a Node isn't either, yet deferring its destruction is fine because the destructor only runs after the grace period, when the object is no longer shared. The surrounding prose is updated to match.

Testing

The example is now a real doctest instead of ignore, so it's compiled and run by CI.

• cargo test --doc guard in crossbeam-epoch — 13 passed (including the new defer_destroy doctest)
• cargo fmt --check -p crossbeam-epoch — clean

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