Document `Image.save_exr()` only being available in editor builds
Document `Image.save_exr()` only being available in editor builds

(cherry picked from commit a1784c64b4b2f65e47927358d30a79ed2c46bcf8)

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