doc warnings about private items

[alamb]

Describe the bug

When I run cargo doc locally I get a bunch of warnings

To Reproduce

cargo doc --document-private-items --no-deps --workspace  

Warnings like

    Checking datafusion-expr v40.0.0 (/Users/andrewlamb/Software/datafusion2/datafusion/expr)
 Documenting datafusion-expr v40.0.0 (/Users/andrewlamb/Software/datafusion2/datafusion/expr)
warning: public documentation for `strip_backtrace` links to private item `Self::BACK_TRACE_SEP`
   --> datafusion/common/src/error.rs:369:66
    |
369 |     /// If backtrace enabled then error has a format "message" [`Self::BACK_TRACE_SEP`] "backtrace"
    |                                                                  ^^^^^^^^^^^^^^^^^^^^ this item is private
    |
    = note: this link resolves only because you passed `--document-private-items`, but will break without
    = note: `#[warn(rustdoc::private_intra_doc_links)]` on by default

warning: `datafusion-common` (lib doc) generated 1 warning

And they point out real problems so ignoring the warnings just means we have less good docs

Expected behavior

I would like cargo doc to run cleanly

Additional context

No response