There's actually more Informix information on the ibm.com site than just the Knowledge Center. For example, there's developerWorks articles, tech docs and notes, and Redbooks. If you want to search all of these sources, just use the search field on the www.ibm.com site. I know it's hard to see, being black on black, but here's what it looks like, in the upper right-hand corner of the webpage:
You don't need to log in. Include the word "informix" in your search and you'll find all sorts of things. The links to documentation in the Knowledge Center look like this: http://www-01.ibm.com/support/knowledgecenter/SSGU8G_12.1.0/com.ibm.welcome.doc/welcome.htm. The number after the uppercase letters is the Informix version number. In this case, 12.1.