Docs: Clarify CanvasItem visibility properties and signals

[AdriaandeJongh]

• Reworded the description of the visible property to emphasize that a CanvasItem is only actually drawn when it and all of its ancestors are visible.
• Appended this important nuance to the visibility_changed and hidden signals, whose descriptions previously did not mention this precise behavior.

[HolonProduction]

Makes sense to clarify this, but I think the ancestor formulation should be changed in some way that excludes Node3D and Ancestors that have a non CanvasItem Node in between. Can't really think of a concise way to say that though. Maybe "CanvasItem ancestors" is sufficient for now 🤷

[Mickeon]

Makes sense to clarify this, but I think the ancestor formulation should be changed in some way that excludes Node3D and Ancestors that have a non CanvasItem Node in between. Can't really think of a concise way to say that though. Maybe "CanvasItem ancestors" is sufficient for now 🤷

I believe struggled with this actually in #93735 . I think the descriptions in this PR are on the right track and far better than currently.

[Mickeon]

Thanks for the feedback! Replacing 'item' with 'node' while subsequently talking about the 'visibility' property is also a bit confusing, so I opted for spelling out [CanvasItem] instead.

I actually don't personally find that to be the case. I have a few notes written about why and how, but in short it makes the description harder to read and localise. I'll consider it a nitpick that I can adjust later, as a lot of the documentation already does this.
To make it less confusing you could've also said "this node", instead.

[akien-mga]

Thanks!

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I merged this locally before seeing the last review comments by @kleonc - those should be addressed in a follow-up.

[AdriaandeJongh]

I merged this locally before seeing the last review comments by @kleonc - those should be addressed in a follow-up.

I'll make a new PR, no problem.