x/tools/cmd/godoc: document as deprecated

[jba]

Document this program as deprecated, and point users to golang.org/x/pkgsite/cmd/pkgsite.

[gopherbot]

Change https://golang.org/cl/349051 mentions this issue: cmd/godoc: deprecate and point to cmd/pkgsite

[gopherbot]

Change https://golang.org/cl/367014 mentions this issue: cmd/godoc: remove extra // characters from deprecation notice