gallium/docs: Make it clear that minIndex, maxIndex is exclusively a index buffer characteristic.

Unlike the indexBias which is specific to a draw call.

This are the semantics of both D3D and GL ARB_draw_elements_base_vertex
extension.

src/gallium/docs/source/context.rst

@@ -156,6 +156,9 @@ as indices to fetch vertex attributes.
 ``indexBias`` is a value which is added to every index read from the index 
 buffer before fetching vertex attributes.
 
+``minIndex`` and ``maxIndex`` describe minimum and maximum index contained in
+the index buffer.
+
 If a given vertex element has ``instance_divisor`` set to 0, it is said
 it contains per-vertex data and effective vertex attribute address needs
 to be recalculated for every index.