net: page_pool: Add documentation on page_pool API

Add documentation explaining the basic functionality and design
principles of the API

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>