docs: net: page_pool: de-duplicate the intro comment

docs: net: page_pool: de-duplicate the intro comment

In commit 82e896d992fa ("docs: net: page_pool: use kdoc to avoid
duplicating the information") I shied away from using the DOC:
comments when moving to kdoc for documenting page_pool API,
because I wasn't sure how familiar people are with it.

Turns out there is already a DOC: comment for the intro, which
is the same in both places, modulo what looks like minor rewording.
Use the version from Documentation/ but keep the contents with
the code.

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://lore.kernel.org/r/20230807210051.1014580-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>