log: mention names of flag constants in {Set,}Flags doc comments
log: mention names of flag constants in {Set,}Flags doc comments

Change-Id: I1217f07530dc7586fd7b933bc6a65bad163782db
Reviewed-on: https://go-review.googlesource.com/c/go/+/47232
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>
Run-TryBot: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,golang/go,akutz/go,CAFxX/go,akutz/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,golang/go,nawawi/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,golang/go,golang/go,akutz/go