DOCS: C4, XY and Block diagrams have special icons with unclear meaning

[viblo-majority]

There are 3 diagrams with special icons next to their names in the documentation. As a user it's not clear what they mean, and I can't see any way to find out.

Maybe it could be fixed with a hover text, or if the reasoning for icon is mentioned on the actual diagram page it could be included there. For example, on C4 there's a info box in the top that says its experimental. If this is the meaning of the two icons then those could be put in that box with the text.