doc: add information about suffix inference to tutorial and manual.

src/test/compile-fail/tutorial-suffix-inference-test.rs

@@ -0,0 +1,22 @@
+fn main() {
+    let x = 3;
+    let y: i32 = 3;
+
+    fn identity_u8(n: u8) -> u8 { n }
+    fn identity_u16(n: u16) -> u16 { n }
+
+    identity_u8(x);  // after this, `x` is assumed to have type `u8`
+    identity_u16(x);
+    //!^ ERROR mismatched types: expected `u16` but found `u8`
+    identity_u16(y);
+    //!^ ERROR mismatched types: expected `u16` but found `i32`
+
+    let a = 3i;
+    
+    fn identity_i(n: int) -> int { n }
+
+    identity_i(a); // ok
+    identity_u16(a); 
+    //!^ ERROR mismatched types: expected `u16` but found `int`
+
+}