docs: Fix return value descriptions for set_virtual_router methods

[michalbil]

Description

Fix return value descriptions for set_virtual_router methods of Interface and AbstractSubinterface classes.

Motivation and Context

Documentation fix.

How Has This Been Tested?

n/a

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes if appropriate.
• [ ] All new and existing tests passed.