My approach would be something like:

1. Get list of all document types
2. Loop through each document type, and do a XPath lookup in the XML cache

In code this would look something like:

IContentTypeService service = UmbracoContext.Current.Application.Services.ContentTypeService;
UmbracoHelper helper = new UmbracoHelper(UmbracoContext.Current);

foreach (IContentType contentType in service.GetAllContentTypes()) {
    <p>@contentType.Alias: @(helper.TypedContentAtXPath("descendant-or-self::" + contentType.Alias + "[@isDoc]").Count())</p>
}

Depending on your XML cache, this might take some time (took a few seconds for a installation with 30k nodes). Also notice that this will only work if all documents are in the XML cache, and it won't look at documents in the recycle bin.