docs: mention static methods

/cc #4217

Author: catamorphism

doc/rust.md

@@ -1195,8 +1195,23 @@ Values with a trait type can have [methods called](#method-call-expressions) on
 for any method in the trait,
 and can be used to instantiate type parameters that are bounded by the trait.
 
-Trait methods may be static.
-Currently, implementations of static methods behave like functions declared in the implementation's module.
+Trait methods may be static,
+which means that they lack a `self` argument.
+This means that they can only be called with function call syntax (`f(x)`)
+and not method call syntax (`obj.f()`).
+The way to refer to the name of a static method is to qualify it with the trait name,
+treating the trait name like a module.
+For example:
+
+~~~~
+trait Num {
+    static pure fn from_int(n: int) -> self;
+}
+impl float: Num {
+    static pure fn from_int(n: int) -> float { n as float }
+}
+let x: float = Num::from_int(42);     
+~~~~
 
 Traits can have _constraints_ for example, in
 

doc/tutorial.md

@@ -2076,6 +2076,32 @@ the preferred way to use traits polymorphically.
 
 This usage of traits is similar to Haskell type classes.
 
+## Static methods
+
+Traits can define _static_ methods, which don't have an implicit `self` argument.
+The `static` keyword distinguishes static methods from methods that have a `self`:
+
+~~~~
+trait Shape {
+    fn area() -> float;
+    static fn new_shape(area: float) -> Shape;
+}
+~~~~
+
+Constructors are one application for static methods, as in `new_shape` above.
+To call a static method, you have to prefix it with the trait name and a double colon:
+
+~~~~
+# trait Shape { static fn new_shape(area: float) -> self; }
+# use float::consts::pi;
+# use float::sqrt;
+struct Circle { radius: float }
+impl Circle: Shape {
+    static fn new_shape(area: float) -> Circle { Circle { radius: sqrt(area / pi) } }
+}
+let s: Circle = Shape::new_shape(42.5);
+~~~~
+
 ## Trait constraints
 
 We can write a trait declaration that is _constrained_ to only be implementable on types that