Display "private fields" instead of "fields omitted"

Also:

  * Always use `/* */` block comments
  * Use the same message everywhere, rather than sometimes prefixing
    with "some"

When I first read rustdoc docs, I was confused why the fields were being
omitted. It was only later that I realized it was because they were
private. It's also always bothered me that rustdoc sometimes uses `//`
and sometimes uses `/*` comments for these messages, so this change
makes them all use `/*`.

Technically, I think fields can be omitted if they are public but
`doc(hidden)` too, but `doc(hidden)` is analogous to privacy. It's
really just used to emulate "doc privacy" when -- because of technical
limitations -- an item has to be public. So I think it's fine to include
this under the category of "private fields".

src/test/rustdoc/toggle-item-contents.rs

@@ -55,7 +55,7 @@ pub union Union {
 
 // @has 'toggle_item_contents/struct.PrivStruct.html'
 // @count - '//details[@class="rustdoc-toggle type-contents-toggle"]' 0
-// @has - '//div[@class="docblock item-decl"]' 'fields omitted'
+// @has - '//div[@class="docblock item-decl"]' '/* private fields */'
 pub struct PrivStruct {
     a: usize,
     b: usize,