PEP 630: Add sections for 2 more gotchas

[encukou]

Two more things to watch out for.

I'm almost ready to convert this PEP into a HOWTO page in the CPython docs. I'd welcome any copy-editing you can throw at it (preferably as a separate PR, which could be rebased onto the docs if the document moves there quickly).
If you don't speak C but want to copyedit, I'd be happy to explain the details.

[CAM-Gerlach]

I'd welcome any copy-editing you can throw at it (preferably as a separate PR, which could be rebased onto the docs if the document moves there quickly).

Indeed; a separate PR would allow easily and consistently copyediting the full document, rather than just the sections added here. Looking over this I actually only spotted one clear issue, a straightforward typo. I'd suggest just fixing that here, and then once this is merged (presumably pretty soon), I can follow up with a copyedit of the whole document as a PR to this repo (for your review) rather than to your branch here, which would make things rather more complicated. Or, I can just go ahead with it now, if these changes won't be merged soon, since these changes are fairly self-contained.

[CAM-Gerlach]

This looks good already; I only noticed one clear issue, a simple typo, which I just made a suggestion for. Otherwise, I can help copyedit the rest of the document in another PR, either before or after this is merged (probably would be better, IMO, to merge this fairly promptly and iterate from there).

[encukou]

Indeed. Thanks!