cmd/benchcmp: mention deprecation in docs

[yi-jiayu]

A deprecation notice was added to benchcmp in a1f8cf0, but there is no mention of it in the benchcmp docs at https://pkg.go.dev/golang.org/x/tools/cmd/benchcmp, so it may not be immediately obvious to new users.

This PR adds the deprecation notice to the documentation as well to make it easier to discover.

[gopherbot]

This PR (HEAD: adc2bec) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/tools/+/255878 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Gobot Gobot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://golang.org/doc/contribute.html#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/255878.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Keith Randall:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/255878.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR (HEAD: 8e19863) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/tools/+/255878 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info

[gopherbot]

Message from Jiayu Yi:

Patch Set 2:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/255878.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Keith Randall:

Patch Set 2: Code-Review+2

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/255878.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR is being closed because golang.org/cl/255878 has been merged.