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

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
-// under) and the tail of a dotted path into it. A non-empty
-// tail is what makes the arm a bound draw: its head is drawn once per expansion
-// (see compileOps).
+// `template.head` resolves and the tail of a dotted path into it. A non-empty tail
+// is what makes the arm a bound draw: its head is drawn once per expansion (see
+// compileOps).
 type arm struct {
 	name   string // as written, for messages
 	key    string