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`.
7e0e07657c