Clarify metadata syntax in manual

Closes #1119

Author: marijnh

doc/tutorial/syntax.md

@@ -295,7 +295,12 @@ is used by the [built-in test framework](test.html)), a name followed
 by `=` and then a literal (as in `#[license = "BSD"]`, which is a
 valid way to annotate a Rust program as being released under a
 BSD-style license), or a name followed by a comma-separated list of
-nested attributes, as in the `cfg` example above.
+nested attributes, as in the `cfg` example above, or in this
+[crate](mod.html) metadata declaration:
+
+    #[link(name = "std",
+           vers = "0.1",
+           url = "http://rust-lang.org/src/std")];
 
 An attribute without a semicolon following it applies to the
 definition that follows it. When terminated with a semicolon, it