This adds a clarification that cogl_vertex_buffer_add can also be
used to replace a previously added attribute with the same name.
* must remain valid until you either call cogl_vertex_buffer_submit() or
* issue a draw call.
*
- * Adds an attribute to a buffer.
+ * Adds an attribute to a buffer, or replaces a previously added
+ * attribute with the same name.
*
* You either can use one of the built-in names such as "gl_Vertex", or
* "gl_MultiTexCoord0" to add standard attributes, like positions, colors