What do you do?
Do you post in the forums?
Do you email email@example.com?
The best way is to click the link which is on the bottom of virtually every page in the MSDN library (thanks for the correction, alimbada) that says: "Send comments about this topic to Microsoft". This will let you email Microsoft directly with comments about a specific topic.
By the way, If you are just interested in giving generic feedback or filing bugs on Microsoft products outside of MSDN, we have a site for that.
You're probably thinking now that this feedback goes into a giant black hole at Microsoft. It does. Just kidding 🙂 But seriously, when the mail is received, the feedback propagates through our publishing system into a database that contains the then anonymized feedback. Systems exist on top of that db that writers can use to gather insights about their topics. For example, one such system tells writers all the feedback that has been written for any pages within their content area ... for all time.
Some teams, such as ours, have an even more in depth methodology for handling this feedback. A very passionate individual on our team combs through all of the feedback for all of the content areas for our team (all the Windows content areas!) and classifies each piece of feedback. IF the feedback is a bug, she files a bug against the owner for the topic area and the owner of that topic will then correct their content as appropriate.
Yeah, you read that right. We read the comments posted to any of our pages on MSDN, even if the comment is "roflcopter lol lol lol" - which we appreciate receiving from time to time, especially if the topic had comments attacking our beloved company or product. Not every writer will respond directly to your feedback, but when you email us, we at the very least see it and consider your suggestions.
So there you have it, you can directly give us documentation feedback through the feedback mechanisms that have existed on MSDN for the past 10 years.
Oh, also of note. If you give feedback, it's best if it is very specific. For example:
"There's a bug on this page: The fSomeNumber parameter is actually a float, NOT a double as the documentation says."
"There's a bug on this page, and it makes me very upset."
Which we typically look at, feel bad about, but can't do very much about.