_content/doc/go1.26.md: add io.ReadAll and fmt.Errorf optimizations

We certainly don't need to document every optimization in the release
notes, but these two CLs I had sent seem more worthwhile to include
given people empirically were avoiding two relatively fundamental
APIs over performance concerns.

For io.ReadAll, people were for example posting things like
"io.ReadAll is documented to be inefficient" on different issues
on various open source projects. See https://go.dev/cl/722500 for
additional details.

For fmt.Errorf, I believe fmt.Errorf("foo") has been considered
by the core Go team to be perfectly fine, but that did not stop a
steady drip of CLs seeking to remove instances for performance reasons,
or linters that complained while citing performance, and so on.
See https://go.dev/cl/708836 for some additional discussion.

Also, it's nice when the simpler and more idiomatic user code has
the same or better performance than more complex code.

Change-Id: I409b495f1cc514588fb2160a7f36cd9bf1ce03e5
Reviewed-on: https://go-review.googlesource.com/c/website/+/729261
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Auto-Submit: David Chase <drchase@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Keith Randall <khr@golang.org>