Consider Optional Config Group and Sets in config doc generation

[machi1990]

#5387 comes with so many goodness like added support for Optional config group and Sets. The config doc generator must also improve in the same sense and must recognise these two constructs.

Quick ideas.

• mark all config items inside a optional group as as optional
• do not display type information for Set but list of or set of ?

/cc @dmlloyd as discussed here #5387 (comment)