I learned in another discussion (answer from Axel Faust) that searching for comments does not work like searching for tags or metadata, because the comments are not part of the documents. You can search for them, but you will find a document-associated discussion and would have to resolve the association to get the according document.
A workaround is to store the content of the comments in a metadata field of the document (i.e. by behaviour)...
A Tag query works like Markus Joos described: TAG:mytag You can try it in the share search - but I think you already know that.