Fix src/libcore/str/mod.rs doc comments
Fix src/libcore/str/mod.rs doc comments

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