Documentation: add missing folio_queue entry

Add missing folio_queue entry.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20241001133920.6e28637b@canb.auug.org.auSigned-off-by: Christian Brauner <brauner@kernel.org>

Documentation/core-api/index.rst

@@ -37,6 +37,7 @@ Library functionality that is used throughout the kernel.
    kref
    cleanup
    assoc_array
+   folio_queue
    xarray
    maple_tree
    idr