Well, when I started learning (around Beta 2, I'm by no means an expert yet but eager to get there!) a lot of the docs about both I/O and type conversions/type casting could've used some work, just to name two features...
And yeah, there's certainly a ton of weak points. We landed a _lot_ of Rustdoc fixes that make certain things better, but there's still a ton more to write. I've been mostly focusing on the longform docs. I just wanted to make sure that I know where people are having problems, so I can know where to focus my efforts. Thanks.
The book is in the "trpl" directory: https://github.com/rust-lang/rust/tree/master/src/doc/trpl
The standard library docs are maintained in doc comments in the appropriate places in the source: https://github.com/rust-lang/rust/tree/master/src/libstd
You can clone the repository, modify it locally, and build and test the docs (yes, the docs get tested, most of the code samples are actually executable tests), or you can just edit them online in GitHub's editor and send a pull request that way if you're just making documentation suggestions that won't need any tests.