Documentation improvements

I'm collecting here various sticking points that I'd like to improve in the documentation, from the perspective of a recent user. The list is WIP.

General improvements

• Document the default values for all settings
• Look for missing settings on webpage (see below)
• Clarify in which order the region/sample suffixes and replacements are applied

Specific options whose documentation could be improved

• Region: Some suffix options are missing from the webpage (e.g. histonamesuff)
• Job: HistoChecks, e.g. describe what it does when NOCRASH and a histo is missing, what can go wrong etc
• Systematic: Difference between OverallUp/down & ScaleUp/down

Typos

• Fit: NLLOffset

Other issues

• #33
• #83
• #123
• #133