doc,cli: clarify --max-old-space-size and --max-semi-space-size units

As far as I understand, both --max-old-space-size and --max-semi-space-size expect mebibytes as the value unit.

This is already clear in the --max-semi-space-size section, as the documentation mentions the shortened MiB unit type.

But it was not clear in --max-old-space-size. The only way to deduce was by reading the example:

On a machine with 2 GiB of memory, consider setting this to 1536 (1.5 GiB)

This PR intends to bring more clarity to the correct unit to use.