Auto merge of #116074 - fzs111:clarify-pin-docs, r=Mark-Simulacrum

Clarify example in `Pin::new_unchecked` docs

This example in the docs of `Pin::new_unchecked` puzzled me for a relatively long time. Now I understand that it comes down to the difference between dropping the `Pin` vs dropping the pinned value.

I have extended the explanation to highlight this difference. In my opinion it is clearer now, and I hope it helps others understand `Pin` better.

Author: bors