net: page_pool: Add page_pool_put_page_bulk() to page_pool.rst

Introduce page_pool_put_page_bulk() entry into the API section of
page_pool.rst

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/a6a5141b4d7b7b71fa7778b16b48f80095dd3233.1606146163.git.lorenzo@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>