Rollup merge of #87279 - sunfishcode:document-unix-argv, r=RalfJung

Add comments explaining the unix command-line argument support.

Following up on #87236, add comments to the unix command-line argument
support explaining that the code doesn't mutate the system-provided
argc/argv, and that this is why the code doesn't need a lock or special
memory ordering.

r? ```@RalfJung```