chore(docs): rename @private to @internal

[alexeagle]

The latter is understood by TypeScript's --stripInternal option, so this lets us
rely more on the tooling provided by typescript team.

Part of #3082

[alexeagle]

cc @IgorMinar @petebacondarwin I've tried to be careful in the docs generation code to replace the right private references, but I'm not confident how to test that the docs are generated the same way. Should I do more careful testing before merging this?

[petebacondarwin]

If you generate the docs in angular.io, both before and after your changes, then you should be able to see if there are any changes via git.

[petebacondarwin]

Hold on, we have migrated all these dgeni packages over to angular.io. You should be making changes there.

[IgorMinar]

as @petebacondarwin said this PR needs to be split into two. one with annotation changes in our code base and then the second one should be for angular.io repo to update the dgeni package.

[IgorMinar]

I'll send a PR removing the dgeni code from our angular/angular

[alexeagle]

the angular.io change is already merged actually (Naomi merged the PR a bit before I was ready...)

[mary-poppins]

Merging PR #4477 on behalf of @alexeagle to branch presubmit-alexeagle-pr-4477.

[alexeagle]

merged in f7aa890

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