logtail,logpolicy: tweak minor cosmetic things

Just reading the code again in prep for some alloc reductions.

Change-Id: I065226ea794b7ec7144c2b15942d35131c9313a8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/buffer.go

@@ -21,6 +21,8 @@ type Buffer interface {
 	TryReadLine() ([]byte, error)
 
 	// Write writes a log line into the ring buffer.
+	//
+	// Write takes ownership of the provided slice.
 	Write([]byte) (int, error)
 }