Red Hat is under that obligation to give out their source code. They are under no obligation to explain it to you. For that, you have to pay them, or figure it out yourself. I have no problem with that model. Why should you feel entitled to their knowledge base or documentation if you haven't paid for it?
What it comes down to is that paywalled documentation is, in my experience, universally inferior to documentation that is publicly available. In my last job I saw "Proprietary Information of <company>" stamped over all the documentation I used, and the thing was the most insulting thing I'd ever seen. The "information" was less useful than the reverse engineering my colleagues had done over the years.
Red Hat's documentation is better now, but documentation tends to rot when people can't see it until they're in the midst of using it. I feel entitled to the knowledge base because I'm not paying for something unless I have a good idea of the sort of issues I'll have when using it, and what the company does to resolve them. Red Hat has made a fine business respecting this up until now.
On the other hand, a lack of publicly available documentation is less important when source is available.
IIRC the GPL does, actually, require you to enumerate your changes to the code. This doesn't necessarily have to be in the form of a diff, though, and can be in a ChangeLog instead.
2. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:
a) You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change.
b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
c) If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.)
Seems like all it requires is that you say "We changed this file, and we changed it on this date." I suppose one could question whether giving detailed information about the changes is simply the "spirit of the rule" or if it's actually "going above and beyond."
