rustdoc: fix doctests with non-feature crate attrs

src/doc/book/documentation.md

@@ -600,7 +600,7 @@ is documented, especially when you are working on a library. Rust allows you to
 to generate warnings or errors, when an item is missing documentation.
 To generate warnings you use `warn`:
 
-```rust
+```rust,ignore
 #![warn(missing_docs)]
 ```
 
@@ -630,7 +630,7 @@ struct Hidden;
 You can control a few aspects of the HTML that `rustdoc` generates through the
 `#![doc]` version of the attribute:
 
-```rust
+```rust,ignore
 #![doc(html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "https://www.rust-lang.org/favicon.ico",
        html_root_url = "https://doc.rust-lang.org/")]

src/doc/book/using-rust-without-the-standard-library.md

@@ -13,7 +13,7 @@ don’t want to use the standard library via an attribute: `#![no_std]`.
 
 To use `#![no_std]`, add it to your crate root:
 
-```rust
+```rust,ignore
 #![no_std]
 
 fn plus_one(x: i32) -> i32 {
@@ -29,7 +29,7 @@ use its features without an explicit import. By the same token, when using
 prelude](../core/prelude/v1/index.html). This means that a lot of code will Just
 Work:
 
-```rust
+```rust,ignore
 #![no_std]
 
 fn may_fail(failure: bool) -> Result<(), &'static str> {