Improve documentation for `String.get_extension()`
Improve documentation for `String.get_extension()`

(cherry picked from commit 09eb98c5306f8d9a17287aa5a7509e17d56f2ee0)

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