Box docs don't mention that you can move out of them

[ExpHP]

I wanted to link somebody to documentation explaining that you can dereference a Box to move the value onto the stack, but... neither the boxed module nor Box mention this or show any examples!

https://doc.rust-lang.org/stable/std/boxed/index.html
https://doc.rust-lang.org/stable/std/boxed/struct.Box.html

[frewsxcv]

Opened a PR for this: #54544