When creating nodes, you can enter a separate title for the menu link. Generally this is useful if your node title is really long, and you want a shorter title to appear in the menu (e.g. node title: 'Our valued clients', menu link title: 'Clients').
Using Views, you can get a node's title very easily. However there is no way of getting its menu link title! I'd like to suggest adding the ability to get a node's 'menu link title' to Views.
Recent comments
There is also Node Comments Search - this searches the node and all comments (default search is node and first page of comments) as part of the content search. Izzy's module above is a separate...
Search does not include comments
I have created a new module comment_search
Search does not include comments
If there is no response, alternatively, you can use a View with a list of comments, with a filter based on the comment text. I haven't tried it myself, I just got an idea.
Search does not include comments