Review feedback for collect_intra_doc_links.rs
Review feedback for collect_intra_doc_links.rs

* Add assertion value is defined.
* Simplify comment.
* Fix bad change in err message.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust