Docs are maintained here: https://github.com/rust-lang/rust/tree/master/src/doc

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.