Right now there's a discussion on Hacker News going on reiterating over Rob Pike's comments on commit messages. There's one particular comment by the user 'mumblemumble' I want to quote for future reference:
Some information is a pre-requisite for understanding a specific block of code. This information is a great fit for comments, because everyone who's looking at the code will need to know it, and having it in a comment ensures that it's discoverable.
Other information is a pre-requisite for understanding the engineering or business decisions that motivated a design decision, but isn't necessary for understanding how the code works or how to interact with it. That makes it less appropriate for comments, because the comment would just be clutter (light pleasure reading at best) under the most typical use case. People who are engaging in those use cases should know how to use git blame to find what they need.
For still others, the unit to which the information applies is not a block of code; it's a set of changes. In those cases, comments are a terrible choice for conveying that information. Maybe a commit message is better. If it's not, you probably need to be looking to a project wiki or some separate documentation files, so that the information can be written and maintained in one place. You can still use comments, but they should merely cross-reference the centralized documentation. Today's copy/paste is tomorrow's outdated and misleading misinformation.