ax(node): replace prose comments with usage examples in bufpool.go

AX Principle 2 — comments must show HOW with real values, not describe
WHAT the signature already says. Replaced three prose descriptions on
getBuffer, putBuffer, and MarshalJSON with concrete call-site examples.

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/bufpool.go

@@ -14,14 +14,15 @@ var bufferPool = sync.Pool{
 	},
 }
 
-// getBuffer retrieves a buffer from the pool.
+// buf := getBuffer()
+// defer putBuffer(buf)
 func getBuffer() *bytes.Buffer {
 	buf := bufferPool.Get().(*bytes.Buffer)
 	buf.Reset()
 	return buf
 }
 
-// putBuffer returns a buffer to the pool.
+// putBuffer(buf) // always called via defer after getBuffer()
 func putBuffer(buf *bytes.Buffer) {
 	// Don't pool buffers that grew too large (>64KB)
 	if buf.Cap() <= 65536 {
@@ -29,8 +30,8 @@ func putBuffer(buf *bytes.Buffer) {
 	}
 }
 
-// MarshalJSON encodes a value to JSON using a pooled buffer.
-// Returns a copy of the encoded bytes (safe to use after the function returns).
+// data, err := MarshalJSON(msg)
+// if err != nil { return nil, err }
 func MarshalJSON(v interface{}) ([]byte, error) {
 	buf := getBuffer()
 	defer putBuffer(buf)