bpf, docs: document open-coded BPF iterators

bpf, docs: document open-coded BPF iterators

Extract BPF open-coded iterators documentation spread out across a few
original commit messages ([0], [1]) into a dedicated doc section under
Documentation/bpf/bpf_iterators.rst. Also make explicit expectation that
BPF iterator program type should be accompanied by a corresponding
open-coded BPF iterator implementation, going forward.

  [0] https://lore.kernel.org/all/20230308184121.1165081-3-andrii@kernel.org/
  [1] https://lore.kernel.org/all/20230308184121.1165081-4-andrii@kernel.org/

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20250509180350.2604946-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>