bpf, docs: BPF Iterator Document

Document that describes how BPF iterators work, how to use iterators,
and how to pass parameters in BPF iterators.

Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Sreevani Sreejith <psreep@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221202221710.320810-2-ssreevani@meta.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>