Respect public-dependency when documenting deps

Fixes #2025

When the public-dependency feature is enabled, cargo doc now only
documents direct dependencies and their public dependencies recursively.
This reduces documentation clutter by excluding transitive private
dependencies that users don't need to know about.

Behavior:
- Without public-dependency: all deps documented (backward compatible)
- With public-dependency enabled:
  * Direct dependencies always documented
  * Transitive deps only if marked public

This allows library authors to control their documentation surface by
marking which dependencies are part of their public API.

Author: Ojus Chugh

src/cargo/core/compiler/unit_dependencies.rs

@@ -637,6 +637,15 @@ fn compute_deps_doc(
     // built. If we're documenting *all* libraries, then we also depend on
     // the documentation of the library being built.
     let mut ret = Vec::new();
+    let is_root = state.ws.is_member(&unit.pkg);
+    
+    // Check if public-dependency feature is enabled
+    let public_dep = state.gctx.cli_unstable().public_dependency
+        || unit.pkg.manifest()
+            .unstable_features()
+            .require(Feature::public_dependency())
+            .is_ok();
+    
     for (id, deps) in state.deps(unit, unit_for) {
         let Some(dep_lib) = calc_artifact_deps(unit, unit_for, id, &deps, state, &mut ret)? else {
             continue;
@@ -657,7 +666,20 @@ fn compute_deps_doc(
             IS_NO_ARTIFACT_DEP,
         )?;
         ret.push(lib_unit_dep);
-        if dep_lib.documented() && state.intent.wants_deps_docs() {
+        
+        // Decide whether to document this dependency.
+        // When public-dependency is enabled, only document:
+        // - Direct dependencies of workspace members
+        // - Public dependencies (recursively)
+        let should_doc = dep_lib.documented() && state.intent.wants_deps_docs() && {
+            if public_dep {
+                is_root || state.resolve().is_public_dep(unit.pkg.package_id(), id)
+            } else {
+                true
+            }
+        };
+        
+        if should_doc {
             // Document this lib as well.
             let doc_unit_dep = new_unit_dep(
                 state,