Update "Local allocation" documentation

[riaqn]

Based on #3475 , and potentially #3471 (in which case, a short sentence will be added to the documentation)

But can be reviewed without merging them.

This PR updates the local allocation documentation by decoupling modes from allocation. In particular, it usese stack_ syntax.

[goldfirere]

Instead of making suggestions here in the PR, I just went ahead and made some edits on top of what you started here, @riaqn. Could you please review my commit? If you're happy with that, then we can merge this.

[riaqn]

Thanks for the edits - they look great. Could you look at my comments below? If you concur, I will make the edits.

[riaqn]

This is blocked by #3471 , because I really want the complete story about stack and primitives.

[riaqn]

We are also blocked by the resolution of #3106

[riaqn]

We are now blocked by the ocamlformat support for #3106 - but it's also fine to tell users to use the old syntax, and fix ocamlformat later.

[goldfirere]

I think we should just try to get this out the door. I don't feel strongly on any of my points, and I think this is a nice improvement over the status quo. Happy with small changes you might make to resolve discussions. Thanks!