x11: Fix comment around modifiers merging, tweak changelog

[bew]

refs: #4151

[bew]

Friendly ping @wez, this is a short comment PR
Build failure is unrelated

[wez]

re: build failure, I think I'll just remove centos 7. It's EOL and not worth the effort to try to keep it going any more.

I was holding off on this PR for the moment, because I'm wondering if unconditionally merging of the modifiers is the cause of #4776 but we haven't had feedback on that yet.

Also, something weird I've noted once or twice in the past week is that if I resize wezterm, sometimes vim mysteriously gets suspend as though I typed ctrl-z. I don't know if this is just some crazy unrelated thing or if there is some weird stuff happening with input on this system--it doesn't seem like resizing with the mouse would be connected to input, but also: it seems weird that the modifiers spontaneously get reset to 0 here. I haven't had time/motivation to sit down and debug it.

[bew]

Your modifiers reset is definitely weird 😬
But this PR doesn't change any code, only makes it clearer imo (by moving & reworking the comment).

I'm thinking your weird behavior could be tracked in a separate issue?
(which might end up reverting / reworking the code for mods merge)