buffer: add comment explaining buffer alignment
buffer: add comment explaining buffer alignment

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

Fixes #3030.

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node