Docs: Proj is not always a data node (#250)

The node recap tables in chapters 4 and 5 classify `Proj` as "Data", but
`ProjNode.isCFG()` has returned true for control projections since chapter 4:

  ch4:  return _idx==0;
  ch5+: return _idx==0 || ctrl() instanceof IfNode;

So `Proj#0` off `Start` is a control node, and from chapter 5 both projections
off an `If` are too -- which is exactly why the dominator walk in
`IfNode.compute()` can step through them.

Change the Type column to "Data / Control" and add a note under each table
pointing at `isCFG()`.  Table alignment is unchanged: "Data / Control" is the
same width as "Abstract class".

Docs-only change.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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