Clearify behaviour of `@export` in combination with setters

[HolonProduction]

This piece of text came up in #9903. When it was added in c41d188838620257c61f21f3b1f23b0b08079623 it wasn't placed under export resources, so it makes sense to move it away from there. Also I changed the wording to something easier to understand (IMO), behavior was tested on 4.3.stable.