Rollup merge of #115476 - RalfJung:abi-compat-docs, r=Mark-Simulacrum

document ABI compatibility

I don't think we have any central place where we document our ABI compatibility rules, so let's create one. The `fn()` pointer type seems like a good place since ABI questions can only become relevant when invoking a function through a function pointer.

This will likely need T-lang FCP.

Author: matthiaskrgr