reflect: document that method sets are lexicographically sorted
reflect: document that method sets are lexicographically sorted

Fixes #30688

Change-Id: I8b63feba4b18bc07a09f6fbfaa33c1b3326b40e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/169597
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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