Added type of return value in VoterInterface.
merged branch richsage/voterinterface-docblock-type (PR #8117)

This PR was submitted for the master branch but it was merged into the 2.1 branch instead (closes #8117).

Discussion
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[Security] Added type of return value in VoterInterface.

| Q             | A
| ------------- | ---
| Fixed tickets | none
| License       | MIT

Simple addition of the return type for the `supportsClass` method, as per the `supportsAttribute` method doc in the interface.

Commits
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c176d1e [Security] Added type of return value in VoterInterface.