Clarify locking and resolving without package name (#7578)

This adds a guiding comment around an hardcoded no-value in the
lock-and-resolve flow.

Ref: https://github.com/astral-sh/uv/pull/7488#issuecomment-2358385418

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv/src/commands/project/lock.rs

@@ -549,6 +549,7 @@ async fn do_lock(
                     .collect(),
                 dev,
                 source_trees,
+                // The root is always null in workspaces, it "depends on" the projects
                 None,
                 Some(workspace.packages().keys().cloned().collect()),
                 &extras,