Document `stats.pprof` config item

Nebula PR: https://github.com/slackhq/nebula/pull/1596

```
  # enables pprof endpoints on the stats listener
  # endpoints: /debug/pprof/, /debug/pprof/cmdline, /debug/pprof/profile,
  #            /debug/pprof/symbol, /debug/pprof/trace
  #pprof: false
```

Added in: ...