Update documentation for programs shown by `bpftool perf list`

[qmonnet]

Command bpftool perf list does not display all perf-related programs. It doesn't show programs attached through BPF links, only those that have an open FD. This should be documented.

In my notes, I also have a mention that writeable tracepoints are not displayed either. Because they use links? Should be investigated and clarified in the documentation.