I can understand internal members being documented - that way it may be easier to browse the documentation for code you're writing within the same assembly. (And there's always InternalsVisibleTo, of course.) For private members, I think it's a bit harder to justify.
If you use Sandcastle to generate offline documentation, you can ask it to generate a new XML file with only the public members - and only the summary parts, at that. I can't remember offhand what it looks like in SHFB, but in our Noda Time project file, I believe this is the relevant section:
<ComponentConfig id="IntelliSense Component" enabled="True">
<component id="IntelliSense Component"
type="SandcastleBuilder.Components.IntelliSenseComponent"
assembly="{@SHFBFolder}SandcastleBuilder.Components.dll">
<output includeNamespaces="false" namespacesFile="Namespaces"
folder="{@OutputFolder}\..\PublicApi" />
</component>
</ComponentConfig>