doc: Make File store/get integer methods clearer
doc: Make File store/get integer methods clearer

Add an example on how to store signed integers on less than 64 bits,
using one bit for the signedness.

(cherry picked from commit cd25d184a5f547d7e7b6332a255908aa00f4ddc7)

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