Use... ...instead of Rationale
"built-in" as an adjective ("built-in function") or "built in"
"builtin" isn't a word
But the manual uses it:
5.48 Object Size Checking Builtins
Actually the whole manual is inconstient:
Atomic Builtins: Built-in functions for atomic memory access.
Object Size Checking: Built-in functions for limited buffer overflow checking.
Other Builtins: Other built-in functions.
Target Builtins: Built-in functions specific to particular targets.
I added you as a CC because you wrote the documentation for the object size checking built ins but I now see this is a much bigger issue than just that one.
I think this should be confirmed as a real issue, even if we don't plan on doing anything right away