Clarify the required child node type in the GridContainer documentation
Clarify the required child node type in the GridContainer documentation

This closes #41822.

(cherry picked from commit 40b5331d794e16c586b9bb5a10a6d820e16be2dc)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot