Allow adding extra documentation to bindgen-generated entities

C libraries are not always very careful about documenting their safety requirements. But unsafe rust functions are expected to document in which circumstances they are safe to use. To resolve this impedance mismatch without huge upstream doc contributions, which may be rejected, it would be nice if one could append manually written # Safety sections to bindgen's autogenerated function docs. This could take the form of a HashMap from C function name to safety docs.

The issue names no files, tests, or entry points, so first locate the part of bindgen that creates documentation for generated entities. Work out how manually supplied safety text keyed by C function name should be configured and appended, then add coverage for the resulting # Safety sections. Done means users can provide these additions without changing the upstream C documentation.