Auto merge of #1039 - fitzgen:more-docs, r=emilio
Auto merge of #1039 - fitzgen:more-docs, r=emilio

book: Clean up the C++ section some more

Trying to set expectations for what we can do with C++ a little better.

r? @emilio

rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen