Auto merge of #132200 - Mark-Simulacrum:strengthen-cross-lang, r=Ralf…

…Jung

Make clearer that guarantees in ABI compatibility are for Rust only

cc rust-lang/rust#132136 (comment) -- it looks like we already had a note that I missed in my initial look here, but this goes further to emphasize the guarantees, including uplifting it to the top of the general documentation.

r? `@RalfJung`