acme: add comment for aligned writes

TBR=r
https://codereview.appspot.com/89510044

src/cmd/acme/exec.c

diff --git a/src/cmd/acme/exec.c b/src/cmd/acme/exec.c
index 0ef6684..6f8bf3f 100644
--- a/src/cmd/acme/exec.c
+++ b/src/cmd/acme/exec.c
@@ -662,6 +662,11 @@
 		warning(nil, "can't create file %s: %r\n", name);
 		goto Rescue1;
 	}
+	// Use bio in order to force the writes to be large and
+	// block-aligned (bio's default is 8K). This is not strictly
+	// necessary; it works around some buggy underlying
+	// file systems that mishandle unaligned writes.
+	// https://codereview.appspot.com/89550043/
 	b = emalloc(sizeof *b);
 	Binit(b, fd, OWRITE);
 	r = fbufalloc();