expert.[hc] Minor changes:

- Fix a few parameter descriptions in expert.h;
- Do whitespace changes;
- Add editor modelines.

Change-Id: I1b75fa88021f1f44bd607cd33d0ab7f9ec672ab7
Reviewed-on: https://code.wireshark.org/review/4122
Reviewed-by: Bill Meier <wmeier@newsguy.com>