message: Add line about 'unused entries'
diff: @@ -165,6 +165,9 @@ where x = len(t[0]) + len(t[1]) + ... + len(t[id-1]) + row
  This can be encoded as `W+1` polynomials in the constraint system, which we
  will reference as `FixedTable(i)` for `0 <= i < W` and `TableID`.
  +For any unused entries, we use `TableID = -1`, to avoid collisions with any
  +tables.
  +
  ### Representing the runtime table
  The runtime table can be considered as a 'mask' that we apply to the fixed
  