all: Add comment to ensure correct atomics alignment (fixes #5813)

Per the sync/atomic bug note:

> On ARM, x86-32, and 32-bit MIPS, it is the caller's
> responsibility to arrange for 64-bit alignment of 64-bit words
> accessed atomically. The first word in a variable or in an
> allocated struct, array, or slice can be relied upon to be
> 64-bit aligned.

All atomic accesses of 64-bit variables in syncthing code base are
currently ok (i.e they are all 64-bit aligned).

Generally, the bug is triggered because of incorrect alignement
of struct fields. Free variables (declared in a function) are
guaranteed to be 64-bit aligned by the Go compiler.

To ensure the code remains correct upon further addition/removal
of fields, which would change the currently correct alignment, I
added the following comment where required:

     // atomic, must remain 64-bit aligned

See https://golang.org/pkg/sync/atomic/#pkg-note-BUG.

lib/stun/stun.go

@@ -39,7 +39,7 @@ const (
 )
 
 type writeTrackingPacketConn struct {
-	lastWrite int64
+	lastWrite int64 // atomic, must remain 64-bit aligned
 	net.PacketConn
 }