Hidden style godoc claims "invisible borders" but corners render as "+"

[Delta456]

Description

types.go documents Hidden as "a box style with invisible borders", but the style renders visible + corner markers:

+            +
 hello hidden
+            +

The + corners match the v2 glyphs, so the v3 doc comment is the newer, incorrect artifact. Changing the glyphs would be a visual breaking change for long-time users; the doc should describe reality (invisible space edges, + corner markers) and show how to get a fully invisible box by overriding the corners with spaces.

Fix prepared locally; will land referencing this issue.