Documented all functions, adding use case and side effects. Also replace the use of shortened argument name by more meaningful ones. Renamed `GPU_batch_instbuf_add_ex` and `GPU_batch_vertbuf_add_ex` to remove the `ex` suffix as they are the main version used (removed the few usage of the other version). Renamed `GPU_batch_draw_instanced` to `GPU_batch_draw_instance_range` and make it consistent with `GPU_batch_draw_range`.