[Navigation] Document 3D support is required for navigation

[AThousandShips]

Should only be cherry picked for 4.2 as this does not apply for 4.1

• Closes: Exporting 2d game with custom godot build that has disabled_3d=yes flag breaks navigation 2d. godot#89185

[mhilbrunner]

It might be worth it to add a single sentence of why this is? The reasoning makes sense to me, but if you are new, this may come as a surprise and seem weird at first glance.

[AThousandShips]

There really isn't a reason as such, see:

• [Navigation] Restore 2D only navigation godot#88681

It was done as the code depends on the 3D side and at the time it failed to compile because of missing guards, so not sure what it should say other than that it depends on the 3D side which isn't enabled

[mhilbrunner]

Well, "because there is no 2D navigation, and everything uses 3D navigation under the hood" IMO makes (some amount of) sense once you know that, and experienced people will probably infer that from that note, but I could see newer people being confused. Maybe not, I guess :)

[mhilbrunner]

Thanks, and thanks for making the change!

[AThousandShips]

Thank you!

[mhilbrunner]

Cherry-picked to 4.2 in #9647.