prefix_command documentation is wrong and should be updated

[mwestphal]

We read the prefix_command documentation:

App *     prefix_command (bool allow=true)
     Do not parse anything after the first unrecognized option and return. 

bool     prefix_command_ {false}
     If true, return immediately on an unrecognized option (implies allow_extras) INHERITABLE. 

There is an incoherence. The method param name is "allow" implying that setting to true would "allow" unrecognized option but the param doc is they other way around.

On testing, it looks like the param doc is incorrect, setting prefix_command to true indeed "allow" to have unrecognized option.