[docs] Update emit cheatsheet

[FarazPatankar]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

• The information currently present in the cheat sheet does not show users how they can emit to a room that is within a namespace.

New behaviour

• Adds an example of how someone would emit to a room within a namespace.

Other information (e.g. related issues)

[darrachequesne]

Thanks!