Cleanup/doc is for use on articles that require major attention. "Major" is of course a subjective word, but, as the wording of the template says, it should be used only when there are multiple issues involved. Perhaps the entire article needs intense copy-editing. Maybe the article is so rife with speculation that it is no longer factual. Of course, "multiple issues" doesn't necessarily mean that it's got more than one of these issues wrong with it. It could just have a lot of one of them.
Note that if the article is merely missing information, Template:Update is perhaps more appropriate. If it's a Doctor Who universe article which has mistakenly adopted an out-of-universe perspective, you should use Template:Oou.
If only a part of an article requires attention, use Template:Section cleanup, instead.
Usage
Type
{{Cleanup|<reason>}}
for example
{{Cleanup|This article displays consistently poor grammar and the plot needs to be sectionalised.}}
at the top of the article requiring attention. It should be obvious to readers that there's a problem with the article. The tag shouldn't be buried at the bottom of the page. Nevertheless, just to display the template on this page:
This article displays consistently poor grammar and the plot needs to be sectionalised.
These problems might be so great that the article's factual accuracy has been compromised. Talk about it here or check the revision history or Manual of Style for more information.
Remember, this template should be used when there are multiple issues, and thus it grammatically expects that you will list more than one reason the article needs a cleanup. If you list only one reason, the resulting sentence will not make sense. That's a little warning to you that you shouldn't be using this template if there's only one problem.