message: fix typos, better documentation
diff: @@ -43,9 +43,10 @@ prefixed by convention with "`--osp:`") are understood:
  `--osp:log-level=<str>` set logging level; valid values (in order of severity)
  are `none`, `error`, `warning`, `info`, and `debug`
  - `--osp:warn-as-error` send `warning` and `error` messages throug the error
  + `--osp:warn-as-error` send `warning` and `error` messages through the error
  callback, otherwise send `warning` messages through
  - the message callback
  + the message callback; must have sufficient `logLevel`
  + to enable warnings
  `--osp:verbose` shortcut for `--osp:log-level=info` and enable debug
  output on `cout`, error output on `cerr`
  @@ -116,9 +117,10 @@ all devices:
  bool debug set debug mode; equivalent to logLevel=2 and numThreads=1
  - bool warnAsError send `warning` and `error` messages throug the error
  + bool warnAsError send `warning` and `error` messages through the error
  callback, otherwise send `warning` messages through
  - the message callback
  + the message callback; must have sufficient `logLevel` to
  + enable warnings
  bool setAffinity bind software threads to hardware threads if set to 1;
  0 disables binding omitting the parameter will let OSPRay
  