Document the fence a calc operand gets, and trim two comments

The one-spelling section described a calc operand only as a route that
renders a level, so a reader could not predict that {..cat.net} beside
{calc(net * 2, 2)} is now a load error with no path token in sight. It says
so, and says what an operand's draw covers and what it leaves free.

Dropped the paragraph in checkBoundLevelsHeld that restated its own first
line and the loop below it, and the escape-analysis note in expand.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

render.go

@@ -138,8 +138,7 @@ func expand(s *session, t *template) string {
 	var b strings.Builder
 	b.Grow(t.grow)
 	// One draw per held name, for this expansion only: a nested template and each
-	// repeat iteration get their own, since each is its own expansion. held reaches
-	// no further than readField, which keeps it and its maps on the stack.
+	// repeat iteration get their own, since each is its own expansion.
 	var held *draws
 	if len(t.held) > 0 {
 		held = &draws{