reflect: linkify the docs for Value.Seq and Value.Seq2

It's unclear to me if we could linkify the mention of
iter.Seq[Value] or improve it in any similar way,
given that it already uses square brackets in a different way.

Change-Id: I8f295c30c17ff6c32bd8e4e06015a8fe85dc2b1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/707935
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>