Documentation never explains what main function's type must be

[catamorphism]

The reference documentation never actually says that an executable program must have a main() function declared, or what main's type must be (that is, no type parameters and either no arguments or a [str] argument).

[Dretch]

In a similar vein, it would also be nice if the compiler says what types are expected when it runs into an invalid type for main.

Right now if the main function is defined as returning int then rustc says:

hello.rs:4:0: 6:1 error: wrong type in main function: found `native fn([str]) -> int`

Perhaps it could say something like:

hello.rs:4:0: 6:1 error: wrong type in main function: found `native fn([str]) -> 
int` but expecting `native fn([str]) -> ()` or `native fn() -> ()`