log: document that log messages end in newlines

Fixes #16564.

Change-Id: Idd7b3c8f1d8415acd952d1efb6dc35ba4191805d
Reviewed-on: https://go-review.googlesource.com/31578
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>

src/log/log.go

@@ -8,6 +8,8 @@
 // Panic[f|ln], which are easier to use than creating a Logger manually.
 // That logger writes to standard error and prints the date and time
 // of each logged message.
+// Every log message is output on a separate line: if the message being
+// printed does not end in a newline, the logger will add one.
 // The Fatal functions call os.Exit(1) after writing the log message.
 // The Panic functions call panic after writing the log message.
 package log