Release script: adjust final messaging

[tbradsha]

When running tools/release-plugin.sh, the final message is this:

	1. Merge the above PR into trunk.
	2. If this is NOT the Jetpack core plugin, the release will shortly be tagged to GitHub and released to SVN.
	3. You can then smoke test the release and use .tools/stable-tag.sh <plugin> to update the stable tag, and you're done!
	4. If you are releasing Jetpack-the-plugin, after the changes make it to the mirror repo, conduct a GitHub release.
	5. Then run ./tools/deploy-to-svn.sh <plugin-name> <tag> to deploy to SVN, smoke test, and flip stable tag.

This is a bit confusing, as there are 5 steps, but step 1 is required, step 2 and 3 is one scenario, and step 4 and 5 are another scenario. It's also quite messy when dealing with an 80-char terminal.

This PR proposes the following instead (open to suggestions):

Next you need to merge the above PR into trunk.

If this is NOT the Jetpack core plugin, the release will shortly be tagged to
GitHub and released to SVN and you can then smoke test the release. Once ready,
use `./tools/stable-tag.sh <plugin>` to update the stable tag, and you're done!

If you are releasing Jetpack (the plugin), wait for the changes to appear in
the mirror repo and conduct a GitHub release. Next, deploy the tag to SVN by
running `./tools/deploy-to-svn.sh <plugin> <tag>`, and smoke test. When ready,
flip the stable tag and you're all set.

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
• [ ] Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

Look at the code.