It’s not convenient to explain punctuation to Siri, but if it’s just occasional, or you don’t have your hands free, these commands can save some time in the editing process.
I think it’s best if you use Siri inside a text editing app like Drafts…It’s easier to edit things there, and sometimes Siri botches things so using the service by pressing and holding the home button still feels a bit risky.
Jordan Shirkman does a fantastic job explaining the best way to use Siri on your iPhone/iPad. He also has the best list I’ve ever seen of punctuation commands for Siri.
Personally, I go back and forth between his method of speaking text into Drafts and just making an audio note in Evernote. I generally try to keep my documents as text, if possible to make sure they’re searchable, but when I need to get an idea out quickly or it’s technical in nature, I find it works better to make them an audio note in Evernote and transcribe later.