memory.c doc: add a note on the 'mixed' memory non-model

[gasche]

This PR takes a description of our current understanding of how to write C code for the OCaml runtime, written jointly with @gadmm in #10992 (comment) , and includes it as a comment in memory.c, as suggested by @OlivierNicole. The purpose is to store our collective knowledge about this tricky issue in a place that is more likely to be found in the future than a comment in an github issue.

(cc also @xavierleroy, @kayceesrk, @jhjourdan who may have opinions about this)

The original message contains a description of "future work" to improve the situation / our current understanding, which I find interesting but did not include in the documentation comment.

How to review?

Please let me know if:

1. you strongly disagree with having such content as a comment in the codebase
2. you disagree with something written in the comment

If neither (1) or (2) hold, then you may want to approve :-)

[xavierleroy]

I'm not sure why I was summoned, but I find this documentation useful, esp. the link to "the Linux memory model", which I hadn't read before. We can always argue what's the best place for such documentation: personally, I believe that comments in an implementation file (.c file) should explain the implementation, not the interface nor the high-level design as in the case here. But there is precedent (the [MM] comment in this file), and it's better than nothing, and it can be changed later if someone bothers to collect all these memory model comments in one place.

[gasche]

I'm not sure why I was summoned, [provides useful feedback and lets the PR make progress towards merging].

Thanks!

[gadmm]

Thanks @gasche