Make ArgGroupSpec.allOptionsNestedExcept public and use it in ManPageGenerator

[remkop]

The fix for #2413 was to create a private method ArgGroupSpec.allOptionsNestedExcept(List).
Keeping the method private allowed this fix to be included in 4.7.8 without breaking the semantic versioning contract.

To allow this method to be used by the ManPageGenerator class, it needs to be public.