Update Pushover docs to include msg.sound

[Paul-Reed]

• Bugfix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds functionality)

Proposed changes

Documenting in the 'Pushover' readme & node info the option to configure the node sound alerts by using msg.sound - a feature which already exists, but not documented.

Checklist

• [x ] I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 66.704% when pulling 8176cd3 on Paul-Reed:master into 15cab5a on node-red:master.