log/slog: clarify Logger.WithGroup doc

log/slog: clarify Logger.WithGroup doc

Make it clear that "qualified by the given name" doesn't imply
any particular implementation, but depends on how the Handler
treats groups.

Updates golang/go#56345.

Change-Id: Idf52553108bc1a7d2091a06cc4fc4cc0fc52cc14
Reviewed-on: https://go-review.googlesource.com/c/go/+/478056
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>