Trim comments that restate or argue

The operand order was written out in three places; it now lives at calcVars,
which fixes it, and the other two point there. Dropped the note explaining
why ')' is not reserved, which said what the set is not rather than what it
is, and shortened reservedList's doc to less than its body. The note on held
now states where it reaches instead of arguing against a design this repo no
longer contains.

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

template.go

@@ -317,9 +317,8 @@ type op struct {
 	r    rune   // kind 'c'
 	arms []arm  // kind 'f': the '|' alternatives, split into key and path once
 	call callFn
-	// operands names the sibling fields a {calc()} reads, in the order its
-	// expression first names them. expand reads them before the call, so the
-	// evaluator never touches the node tree. nil for every other builtin.
+	// operands names the sibling fields a {calc()} reads, in the order calcVars
+	// fixed; expand reads them before the call. nil for every other builtin.
 	operands []string
 }