slog-handler-guide: warn against embedding

For golang/go#73057.

Change-Id: Id9ee1c0af03f9b49f6063aa946b6fe5cfbc02483
Reviewed-on: https://go-review.googlesource.com/c/example/+/660956
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: jba

slog-handler-guide/guide.md

@@ -94,6 +94,11 @@ so it will sometimes produce invalid YAML.
 For example, it doesn't quote keys that have colons in them.
 We'll call it `IndentHandler` to forestall disappointment.
 
+A brief aside before we start: it is tempting to embed `slog.Handler` in your
+custom handler and implement only the methods that you need.
+Loggers and handlers are too tightly coupled for that to work. You should
+implement all four handler methods.
+
 We begin with the `IndentHandler` type
 and the `New` function that constructs it from an `io.Writer` and options: