Document the types of "static" functions

[Mark-Simulacrum]

refiling rust-lang/rust#38211

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.

[Havvy]

The proper section is function-item-types.

They all do implement Clone now.