Next meetings macro parameters descriptions don't provide guidance for filling in the values

The next meetings macro "boolean" parameters actually accept "show" and "hide" as values, but this is not written in the parameter description.

It can be "guessed' from the default values of these parameters ("hide"), but if meeting application is used in french for french users, guessing will work less well.
It should be a rule to document possible values of the parameters of macros, until there is support for lists of values for wiki macros ( XWIKI-13282 ).