Document that CStrings live in the libc heap

Insists on the fact that the memory is managed by malloc
and not jemalloc
Closes #17067
Document that CStrings live in the libc heap

Closes issue #17067 and improves pr #17355
Doc says to avoid mixing allocator instead of forbiding it
auto merge of #17355 : gamazeps/rust/issue17210, r=alexcrichton

Insists on the fact that the memory is managed by malloc
and not jemalloc
Closes #17067