Update no_std docs

This is part of #28572, but doesn't complete it. Amongst other things,
this patch:

* Increases consistency in the way feature flags are used with other
  docs.
* Removes the ignores, which is nice: we actually had some syntax errors
  in the examples 😭.
* Mentions #![no_core]

Realistically, this document used to be in the order of least to most:
nothing, then adding core. But with the changes in RFC 1184, this is
backwards: it now shows stuff that uses core from the beginning. In the
future, I'd like to revamp this to go from 'most to least', but I'd like
to see the discussion in https://github.com/rust-lang/rust/issues/27701
goes before I write more.

src/doc/trpl/no-stdlib.md

@@ -4,14 +4,6 @@ By default, `std` is linked to every Rust crate. In some contexts,
 this is undesirable, and can be avoided with the `#![no_std]`
 attribute attached to the crate.
 
-```ignore
-// a minimal library
-#![crate_type="lib"]
-#![feature(no_std)]
-#![no_std]
-# // fn main() {} tricked you, rustdoc!
-```
-
 Obviously there's more to life than just libraries: one can use
 `#[no_std]` with an executable, controlling the entry point is
 possible in two ways: the `#[start]` attribute, or overriding the
@@ -21,7 +13,10 @@ The function marked `#[start]` is passed the command line parameters
 in the same format as C:
 
 ```rust
-#![feature(lang_items, start, no_std, libc)]
+# #![feature(libc)]
+#![feature(lang_items)]
+#![feature(start)]
+#![feature(no_std)]
 #![no_std]
 
 // Pull in the system libc library for what crt0.o likely requires
@@ -47,11 +42,13 @@ with `#![no_main]` and then create the appropriate symbol with the
 correct ABI and the correct name, which requires overriding the
 compiler's name mangling too:
 
-```ignore
+```rust
+# #![feature(libc)]
 #![feature(no_std)]
+#![feature(lang_items)]
+#![feature(start)]
 #![no_std]
 #![no_main]
-#![feature(lang_items, start)]
 
 extern crate libc;
 
@@ -92,19 +89,24 @@ instead.
 
 The core library has very few dependencies and is much more portable than the
 standard library itself. Additionally, the core library has most of the
-necessary functionality for writing idiomatic and effective Rust code.
+necessary functionality for writing idiomatic and effective Rust code. When
+using `#![no_std]`, Rust will automatically inject the `core` crate, just like
+we do for `std` when we’re using it.
 
 As an example, here is a program that will calculate the dot product of two
 vectors provided from C, using idiomatic Rust practices.
 
-```ignore
-#![feature(lang_items, start, no_std, core, libc)]
+```rust
+# #![feature(libc)]
+#![feature(lang_items)]
+#![feature(start)]
+#![feature(no_std)]
+#![feature(core)]
+#![feature(core_slice_ext)]
+#![feature(raw)]
 #![no_std]
 
-# extern crate libc;
-extern crate core;
-
-use core::prelude::*;
+extern crate libc;
 
 use core::mem;