Document the types of "static" functions

[jrvidal]

There are a few aspects of "static" (non-closure) functions that I find useful to know but I can't seem to locate in the docs:

• They all implement Copy.
  • And they don't implement Clone (is this unusual?)
• They automatically implement the respective function call traits Fn, FnMut and FnOnce.
  • And since they don't have an environment... they are equivalent?

I think it might be appropriate to have an entry in the std/core documentation with this. Not sure if a new "primitive" page would be appropriate for it.

[bluss]

The lack of Clone is a rather annoying bug (#28229) and does of course makes no sense to keep around (Copy requires that Clone is implemented according to its definition).

[Mark-Simulacrum]

Tagging with T-doc. cc @rust-lang/docs, maybe this is good for inclusion into the reference? Or the book?

[Mark-Simulacrum]

This should be refiled onto the reference https://github.com/rust-lang-nursery/reference. Closing.