testing/slogtest: clarify that passed handler must be enabled at info

Fixes #62115.

Change-Id: I1ecd3048ccbf2014bc1d561c7cd79a0c2fe8e4bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/520676
Run-TryBot: Jonathan Amsterdam <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: jba

src/testing/slogtest/slogtest.go

@@ -36,6 +36,7 @@ type testCase struct {
 //
 // TestHandler installs the given Handler in a [slog.Logger] and
 // makes several calls to the Logger's output methods.
+// The Handler should be enabled for levels Info and above.
 //
 // The results function is invoked after all such calls.
 // It should return a slice of map[string]any, one for each call to a Logger output method.