slog: clarify the responsibilities of Handler methods
slog: clarify the responsibilities of Handler methods

Change-Id: I2c302b3af61965387fbc927937fd98dc1dcbb0fc
Reviewed-on: https://go-review.googlesource.com/c/exp/+/429840
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/exp,golang/exp,golang/exp