Expand `cold` documentation.

[ehuss]

Should there be more documentation about what the cold attribute does? Or is
that an implementation detail? Its effects depend on the backend in use.

• It reduces the MIR inlining threshold.
• The LLVM backend: "This calling convention attempts to make code in the caller as
  efficient as possible under the assumption that the call is not commonly
  executed. As such, these calls often preserve all registers so that the
  call does not break any live ranges in the caller side."

One option is to include the information in a note block making it clear it is rustc behavior.