adds comment explaining u3a_minimum
# define u3a_bytes (c3_w)((1 << (2 + u3a_bits)))
/* u3a_minimum: minimum number of words in a box.
+ **
+ ** wiseof(u3a_cell) + wiseof(u3a_box) + 1 (trailing siz_w)
*/
#ifdef U3_MEMORY_DEBUG
# define u3a_minimum 8
