docs($resource): describe your change...

[coderbydesign]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Document update.

What is the current behavior? (You can also link to an open issue here)
The documentation currently reads a bit confusing for $resource actions/cache option.

What is the new behavior (if this is a feature change)?
Add a minor clarification in cache option.

Does this PR introduce a breaking change?
No.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[coderbydesign]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!