Document that CStrings live in the libc heap

Insists on the fact that the memory is managed by malloc
and not jemalloc
Closes #17067
The default memory allocator is not always jemalloc - it's an implementation detail. It shouldn't be mentioned in any of the documentation like this. The libc symbols may also be resolving to symbols provided by jemalloc, so it's not accurate to explain it this way. The only thing the documentation should say is that it's not well defined to mix the C allocator API with the Rust allocator API.