rustdoc: Add explanation when linting against public to private item links

The additional note helps explaining why the lint was triggered and that
--document-private-items directly influences the link resolution.

Author: dennis-hamester

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -1076,6 +1076,13 @@ fn privacy_error(
         if let Some(sp) = sp {
             diag.span_label(sp, "this item is private");
         }
+
+        let note_msg = if cx.render_options.document_private {
+            "this link resolves only because you passed `--document-private-items`, but will break without"
+        } else {
+            "this link will resolve properly if you pass `--document-private-items`"
+        };
+        diag.note(note_msg);
     });
 }
 

src/test/rustdoc-ui/intra-links-private.private.stderr

@@ -5,6 +5,7 @@ LL | /// docs [DontDocMe]
    |           ^^^^^^^^^ this item is private
    |
    = note: `#[warn(intra_doc_link_resolution_failure)]` on by default
+   = note: this link resolves only because you passed `--document-private-items`, but will break without
 
 warning: 1 warning emitted
 

src/test/rustdoc-ui/intra-links-private.public.stderr

@@ -5,6 +5,7 @@ LL | /// docs [DontDocMe]
    |           ^^^^^^^^^ this item is private
    |
    = note: `#[warn(intra_doc_link_resolution_failure)]` on by default
+   = note: this link will resolve properly if you pass `--document-private-items`
 
 warning: 1 warning emitted
 

src/test/rustdoc-ui/issue-74134.private.stderr

@@ -5,6 +5,7 @@ LL |     /// [`PrivateType`]
    |          ^^^^^^^^^^^^^ this item is private
    |
    = note: `#[warn(intra_doc_link_resolution_failure)]` on by default
+   = note: this link resolves only because you passed `--document-private-items`, but will break without
 
 warning: 1 warning emitted
 

src/test/rustdoc-ui/issue-74134.public.stderr

@@ -5,6 +5,7 @@ LL |     /// [`PrivateType`]
    |          ^^^^^^^^^^^^^ this item is private
    |
    = note: `#[warn(intra_doc_link_resolution_failure)]` on by default
+   = note: this link will resolve properly if you pass `--document-private-items`
 
 warning: 1 warning emitted