Option docs conflates panics with safety

[bluss]

In Rust, safety is memory safety, and panic is safe. Option docs should not use "safety" for this notice: https://doc.rust-lang.org/nightly/std/option/enum.Option.html#safety-note

[sfackler]

Should probably just retitle that section to "Warning" or "Note" or something like that.

[steveklabnik]

Yes, this seems sub-par. I would remove the heading and move it into the regular description, personally. If anyone would like to fix this, I would be happy to mentor.

[jpadkins]

I'd be interested in fixing this @steveklabnik

To clarify:

• remove the Safety note header
• move the text In general, because... to be directly under the unwrap definition,
  so that it's right after the text Moves the value...