That might be something that have changed between Qt versions. You should go to the bug report system and see if there's already something about it. If not please consider opening a bug for the documentation to be updated on that aspect.
@vishnu Unfortunately I too haven't used the tool myself. Perhaps you to create your own qdocconf configuration file depending upon your requirement. The docs seems to be explaining all the required stuff. Try out the sample conf file mentioned on that page. The tool can be run from anywhere as long as the tool path and conf file path and project paths are valid.
I don't think the fix for this situation is sprinkling the docs with "is always greater than or equal to 0". I don't really think there is a doubt in someones head about whether or not a size or length function returns negative value no matter if it's unsigned int, qint64 or size_t.
I think the opposite is more reasonable - document cases where it can actually return negative value and what it means, which Qt docs are doing pretty consistently (correct me if I'm wrong).
A featrure as you request may be latest with larger projects too much for the debugger to handle.
The closest to get such a behaviour is to use F11 and Shift+F11. F11 will enter every possible method and Shift+F11 will help you to run the remainder of the method. IMO sufficient enough to get a really detailed anaylsis of source code.