Leave an arm's key to template.head's doc, and reflow the arm comment

hold.go

@@ -20,8 +20,7 @@ type draw struct {
 	null bool
 }
 
-// readField renders one arm of a token. An arm's key is a sibling field or a
+// readField renders one arm of a token. A name the expansion holds — a level some
-// reference linkRefs bound into refHeads. A name the expansion holds — a level some
 // token addresses by dotted path, or a field an operand reads — is drawn once and
 // kept, so {place.postal-code} and {place.locality} read one row, either read twice
 // gives one value, and a shown operand is the operand computed. Every other name is

template.go

@@ -268,10 +268,9 @@ func fieldTokens(format string) []string {
 }
 
 // arm is one alternative of a {a|b} token or one operand, split into the key
-// `template.head` resolves (a sibling field, or the head a reference is bound
+// `template.head` resolves and the tail of a dotted path into it. A non-empty tail
-// under) and the tail of a dotted path into it. A non-empty
+// is what makes the arm a bound draw: its head is drawn once per expansion (see
-// tail is what makes the arm a bound draw: its head is drawn once per expansion
+// compileOps).
-// (see compileOps).
 type arm struct {
 	name   string // as written, for messages
 	key    string