gmenumodel: Acknowledge in docs that the UI used in the example is old

Philip Withnallrequested to merge
pwithnall/glib:3451-ui-example into main

This kind of deeply nested menu is definitely no longer good UI practice for most apps (deeply nested menus make things hard to find, and require good mouse control to navigate). However, it does serve as a good demonstration of the concepts in GMenuModel, so keep it, with a sentence to acknowledge that it’s not a good UI.

Signed-off-by: Philip Withnall pwithnall@gnome.org

Fixes: #3451 (closed)

Closes #3451 (closed)

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