Improve ptr::read docs

[steveklabnik]

Fixes #21491

r? @kmcallister @gankro

[Gankra]

@bors r+ ccb5 rollup

[kmcallister]

How about "...leaves the memory in src unchanged, but runs the destructor"? I think that's an important part.

[Gankra]

With non-zeroing drop, this would be an accurate description of a normal drop, no?

[huonw]

@kmcallister it doesn't run the destructor

[kmcallister]

@huonw: How else do you explain the behavior of my test program in #21491? afaik, ptr::read is the standard way to explicitly call a destructor in Rust.

[huonw]

The value is memcpy'd onto the stack and ignored, so the destructor is run there. Something like let x: T = ptr::read(some_ptr); runs no destructors: just copies the value into x on the stack.

[kmcallister]

Fair enough, I see what you mean. I feel like the idiom of discarding ptr::read(some_ptr) to run a destructor is too confusing if we don't mention it here, but I'm not sure.

[steveklabnik]

I'm happy to adapt this to whatever everyone decides is best :)

[Gankra]

How about "Copies the value behind the pointer regardless of whether it is Copy" or something to that effect.