GPU: Cleanup GPU_batch.h documentation and some of the API for consistency

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`.

source/blender/python/gpu/gpu_py_batch.c

@@ -157,7 +157,7 @@ static PyObject *pygpu_batch_vertbuf_add(BPyGPUBatch *self, BPyGPUVertBuf *py_bu
   PyList_Append(self->references, (PyObject *)py_buf);
 #endif
 
-  GPU_batch_vertbuf_add(self->batch, py_buf->buf);
+  GPU_batch_vertbuf_add(self->batch, py_buf->buf, false);
   Py_RETURN_NONE;
 }