Document what the type of the main() function should be

Author: catamorphism

doc/rust.md

@@ -550,6 +550,9 @@ location of which -- in the module tree of the current crate -- is defined
 from outside the source file: either by an explicit `source_directive` in
 a referencing crate file, or by the filename of the source file itself.
 
+A source file that contains a `main` function can be compiled to an
+executable. If a `main` function is present, it must have no [type parameters](#type-parameters)
+and no [constraints](#constraints). Its return type must be [`nil`](#primitive-types) and it must either have no arguments, or a single argument of type `[str]`.
 
 # Items and attributes
 
@@ -592,6 +595,8 @@ the name of the enclosing item, or is private to the enclosing item (in the
 case of functions). The exact locations in which sub-items may be declared is
 given by the grammar.
 
+### Type Parameters
+
 All items except modules may be *parametrized* by type. Type parameters are
 given as a comma-separated list of identifiers enclosed in angle brackets
 (`<...>`), after the name of the item and before its definition. The type