Revert "sync: dropped "NOTICE: --use-superproject is in beta ..." message."
Revert "sync: dropped "NOTICE: --use-superproject is in beta ..." message."

This reverts commit d53cb9549a0c57939b12c5c35a6b581aec2ca36f. As long as
repo's reference docs treat this feature as a work in progress and don't
cover it well enough to allow all repo maintainers to easily support it,
it is inconsistent to report to users that it is no longer in beta.
Thanks for 8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com for noticing.

https://crbug.com/gerrit/15527 tracks the required documentation changes
before we'd be ready to roll forward again.

Change-Id: Ic9bd951cfb3c1abf6e1bfa30dfe4afa1c9b7bec6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/327337
Reviewed-by: Jonathan Nieder <96e93edf89a1b75e35c5d08e01778cfc4fff73d9@google.com>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>
Tested-by: Jonathan Nieder <96e93edf89a1b75e35c5d08e01778cfc4fff73d9@google.com>

GerritCodeReview/git-repo,couchbasedeps/git-repo,lewixliu/git-repo,GerritCodeReview/git-repo,couchbasedeps/git-repo,lewixliu/git-repo