You can always include a comment in the wiki markup. Comments are not shown on the page itself, but are visible when editing. Comments begin with <!-- and end with -->
Example:
<!-- This is a comment that will be displayed while editing, but not visible in the page itself. You can use this to leave notes for editors. -->
If you see typos by a different user on a talk page, it's usually poor etiquette to modify someone else's comments. A talk page is used for discussion about a page, and are generally held to different standards compared to mainspace pages.