docs($injector): Reword fn param docs and link to DI information

[caitp]

The documentation for $injector.invoke is currently very unclear, and contains a typographic error.

I'd like to link to the more detailed DI docs instead of simply stating "function annotation" as though that means something out of context.

---

note: I'm not sure if I need to specify this link as an external link or not. If it's possible, please let me know so I can correct this.

[mary-poppins]

Thanks for the PR!

• Contributor signed CLA now or in the past
  • If you just signed, leave a comment here with your real name
• PR's commit messages follow the commit message format

If you need to make changes to your pull request, you can update the commit with git commit --amend.
Then, update the pull request with git push -f.

Thanks again for your help!

[caitp]

I've signed the CLA previously for #4078

Build failure appears to be unrelated to these changes -- Also FF saucelabs build didn't start.

[btford]

Yep, ignore the travis failure.

[btford]

You don't have to specify the full path in your link. I took care of making that little change.

Landed as ba38de2. Thanks!