maple_tree: update the documentation of maple tree

Introduce the new interface mtree_dup() in the documentation.

Link: https://lkml.kernel.org/r/20231027033845.90608-7-zhangpeng.00@bytedance.com
Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Mike Christie <michael.christie@oracle.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Peng Zhang

Documentation/core-api/maple_tree.rst

@@ -81,6 +81,9 @@ section.
 Sometimes it is necessary to ensure the next call to store to a maple tree does
 not allocate memory, please see :ref:`maple-tree-advanced-api` for this use case.
 
+You can use mtree_dup() to duplicate an entire maple tree. It is a more
+efficient way than inserting all elements one by one into a new tree.
+
 Finally, you can remove all entries from a maple tree by calling
 mtree_destroy().  If the maple tree entries are pointers, you may wish to free
 the entries first.
@@ -112,6 +115,7 @@ Takes ma_lock internally:
  * mtree_insert()
  * mtree_insert_range()
  * mtree_erase()
+ * mtree_dup()
  * mtree_destroy()
  * mt_set_in_rcu()
  * mt_clear_in_rcu()