Improve documentation for OutputService#createSymlinkTree.

Document that the map may contain null values to represent empty files and link to the relevant python logic.

Note that the special handling for `__init__.py` files could be cleaned up if #10076 is fixed.

PiperOrigin-RevId: 644090954
Change-Id: I8b402059125a52e27615ddbd559c5aa538c2d6ff

src/main/java/com/google/devtools/build/lib/vfs/OutputService.java

@@ -151,16 +151,17 @@ void finalizeAction(Action action, OutputMetadataStore outputMetadataStore)
   @Nullable
   BatchStat getBatchStatter();
 
-  /**
-   * @return true iff createSymlinkTree() is available.
-   */
+  /** Returns true iff {@link #createSymlinkTree} is available. */
   boolean canCreateSymlinkTree();
 
   /**
-   * Creates the symlink tree
+   * Creates a symlink tree.
    *
-   * @param symlinks the symlinks to create
-   * @param symlinkTreeRoot the symlink tree root, relative to the execRoot
+   * @param symlinks map from {@code symlinkTreeRoot}-relative path to symlink target; may contain
+   *     null values to represent an empty file instead of a symlink (can happen with {@code
+   *     __init__.py} files, see {@link
+   *     com.google.devtools.build.lib.rules.python.PythonUtils.GetInitPyFiles})
+   * @param symlinkTreeRoot the symlink tree root, relative to the exec root
    * @throws ExecException on failure
    */
   void createSymlinkTree(Map<PathFragment, PathFragment> symlinks, PathFragment symlinkTreeRoot)