Address review comments.

Author: ehuss

src/items/extern-crates.md

@@ -83,11 +83,12 @@ by using an underscore with the form `extern crate foo as _`. This may be
 useful for crates that only need to be linked, but are never referenced, and
 will avoid being reported as unused.
 
-The `#[macro_use]` attribute will work as usual and import the macro names
+The [`#[macro_use]` attribute] will work as usual and import the macro names
 into the macro-use prelude.
 
 [IDENTIFIER]: identifiers.html
 [RFC 940]: https://github.com/rust-lang/rfcs/blob/master/text/0940-hyphens-considered-harmful.md
+[`#[macro_use]` attribute]: attributes.html#macro-related-attributes
 [`alloc`]: https://doc.rust-lang.org/alloc/
 [`crate::`]: paths.html#crate
 [`proc_macro`]: https://doc.rust-lang.org/proc_macro/

src/items/use-declarations.md

@@ -180,6 +180,21 @@ fn main() {
 }
 ```
 
+The unique, unnameable symbols are created after macro expansion so that
+macros may safely emit multiple references to `_` imports. For example, the
+following should not produce an error:
+
+```rust
+macro_rules! m {
+    ($item: item) => { $item $item }
+}
+
+m!(use std as _;);
+// This expands to:
+// use std as _;
+// use std as _;
+```
+
 [IDENTIFIER]: identifiers.html
 [_SimplePath_]: paths.html#simple-paths
 [`extern crate`]: items/extern-crates.html