Update docs to include Sized trait, which is needed

Author: nikomatsakis

src/doc/guide-unsafe.md

@@ -461,11 +461,12 @@ fn start(_argc: int, _argv: *const *const u8) -> int {
     0
 }
 
-// These functions are invoked by the compiler, but not
+// These functions and traits are used by the compiler, but not
 // for a bare-bones hello world. These are normally
 // provided by libstd.
 #[lang = "stack_exhausted"] extern fn stack_exhausted() {}
 #[lang = "eh_personality"] extern fn eh_personality() {}
+#[lang = "sized"] trait Sized { }
 # // fn main() {} tricked you, rustdoc!
 ```
 
@@ -488,13 +489,14 @@ pub extern fn main(argc: int, argv: *const *const u8) -> int {
 
 #[lang = "stack_exhausted"] extern fn stack_exhausted() {}
 #[lang = "eh_personality"] extern fn eh_personality() {}
+#[lang = "sized"] trait Sized { }
 # // fn main() {} tricked you, rustdoc!
 ```
 
 
 The compiler currently makes a few assumptions about symbols which are available
 in the executable to call. Normally these functions are provided by the standard
-library, but without it you must define your own.
+xlibrary, but without it you must define your own.
 
 The first of these two functions, `stack_exhausted`, is invoked whenever stack
 overflow is detected.  This function has a number of restrictions about how it
@@ -508,6 +510,12 @@ mechanisms of the compiler. This is often mapped to GCC's personality function
 information), but crates which do not trigger failure can be assured that this
 function is never called.
 
+The final item in the example is a trait called `Sized`. This a trait
+that represents data of a known static size: it is integral to the
+Rust type system, and so the compiler expects the standard library to
+provide it. Since you are not using the standard library, you have to
+provide it yourself.
+
 ## Using libcore
 
 > **Note**: the core library's structure is unstable, and it is recommended to
@@ -686,6 +694,7 @@ fn main(argc: int, argv: *const *const u8) -> int {
 
 #[lang = "stack_exhausted"] extern fn stack_exhausted() {}
 #[lang = "eh_personality"] extern fn eh_personality() {}
+#[lang = "sized"] trait Sized {}
 ```
 
 Note the use of `abort`: the `exchange_malloc` lang item is assumed to