log/slog: rearrange package doc

log/slog: rearrange package doc

Move the "Attrs and Values" section lower. It describes an optimization;
the API it covers is not essential.

Also, move the brief section on Logger.With up to the first section.
It was in the "Groups" section but didn't belong there.

Change-Id: I0e36ef654e95f918d5b480566ec58d9990d26b40
Reviewed-on: https://go-review.googlesource.com/c/go/+/487856
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>