net: netmem: document netmem and memory provider design in comments

Clarify the netmem, memory provider, page_pool, and skb fragment design
principles in header and code comments:

- Memory providers allocate struct net_iov or struct page, cast them to
  netmem_ref, and pass them to page_pool; page_pool, drivers, and the
  core stack operate on netmem_ref and must not downcast to page or
  net_iov outside dedicated netmem helpers.
- While current memory providers supply struct net_iov, they are not
  architecturally restricted to net_iov and may supply page-backed
  netmems.
- While current net_iov types are CPU-unreadable, net_iov has no
  inherent restrictions and may be readable or unreadable.
- New code should generalize existing limitations as much as possible to
  match these design principles.
- Per-provider logic belongs in memory_provider_ops, and per-netmem-type
  logic belongs in netmem helpers.
- All frags in an skb must either all be struct page-backed or all
  belong to the same memory provider instance; mixing them (including
  via coalescing) is not allowed.

Cc: Luigi Rizzo <lrizzo@google.com>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20261010033630.1171692-2-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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