Elaborate on rustdoc example reason for being ignored.

Author: ehuss

src/doc/rustdoc/src/advanced-features.md

@@ -47,15 +47,15 @@ all type errors and name resolution errors with function bodies. Note that this
 work for anything outside a function body: since Rustdoc documents your types, it has to
 know what those types are! For example, this code will work regardless of the platform:
 
-```rust,ignore (platform-specific)
+```rust,ignore (platform-specific,rustdoc-specific-behavior)
 pub fn f() {
     use std::os::windows::ffi::OsStrExt;
 }
 ```
 
 but this will not, because the unknown type is part of the function signature:
 
-```rust,ignore (platform-specific)
+```rust,ignore (platform-specific,rustdoc-specific-behavior)
 pub fn f() -> std::os::windows::ffi::EncodeWide<'static> {
     unimplemented!()
 }