buffer: add comment explaining buffer alignment

Avoids alignment issues (unaligned loads/stores) on ARM. Originally
added in commit 285d8c6.

Fixes #3030.

lib/buffer.js

@@ -199,8 +199,8 @@ function Buffer(subject, encoding, offset) {
       if (!pool || pool.length - pool.used < this.length) allocPool();
       this.parent = pool;
       this.offset = pool.used;
-      pool.used += this.length;
-      if (pool.used & 7) pool.used = (pool.used + 8) & ~7;
+      // Align on 8 byte boundary to avoid alignment issues on ARM.
+      pool.used = (pool.used + this.length + 7) & ~7;
 
     } else {
       // Zero-length buffer