Expand iterator program docs

[dylandreimerink]

We briefly mention iterator programs in https://docs.ebpf.io/linux/program-type/BPF_PROG_TYPE_TRACING/#iterator but what we currently have does not do it justice. This section could use some attention. In particular an enumeration of the different things we can iterate over, and the special way in which iterators are actually used (essentially custom pseudo files where contents are generated by eBPF).

https://lore.kernel.org/bpf/20200509175859.2474608-1-yhs@fb.com/
https://lore.kernel.org/bpf/20200723184108.589857-1-yhs@fb.com/

[karthikmanam]

@dylandreimerink Can I work on this? Please assign this to me.