1202: doc(runtime-c-api) Improve documentation of the `memory` module r=syrusakbary a=Hywan
Address https://github.com/wasmerio/wasmer/issues/1203.
As part of the daily doc routine, this patch improves the documentation of `memory` module.
This patch also handles more null pointers.
Co-authored-by: Ivan Enderlin <ivan.enderlin@hoa-project.net>