DOC: config: unify sample conv|fetches optional arguments syntax

DOC: config: unify sample conv|fetches optional arguments syntax

The most common way (and proper way it seems) to declare optional
arguments in sample fetch or converters' prototype is to declare
them between square brackets, including the leading coma (because the
coma should be omitted if the argument is not provided). Also, when
multiple optional arguments are found, we should apply the same logic
but recursively.

In this patch we fix prototypes that include optional arguments and don't
follow this syntax. This improves readibility and sets the norm for
upcoming sample fetches/converters.