GDScript: Clarify setter/getter documentation

[dalexeev]

• Closes Exported variables don't use their setters on initialisation godot#62739.
• Closes Confusing syntax error when using type hint for property setter godot#67341.
• Closes Export var not set when use setget godot#78862.
• Also clarifies the wording about sharing a setter/getter between multiple variables, see GDScript: property setters do not work with curried lambda functions godot#70448.

[mhilbrunner]

Whoops, this one kinda slipped through. LGTM, merged, thank you! 🎉 In the future, you're welcome to ping people on the docs team like me here or in the #docs channel on Rocketchat if you don't get any reaction or review at all after like, a week or so :)

[mhilbrunner]

Cherry-picked to 4.1.