Document DirectGrammar correctly

[alexanderkoller]

The patterns in a DirectGrammar can be anything that can be on the RHS of a JSGF rule, as long as there are no nonterminals. See the discussion in #109 .

Document this precisely in https://github.com/dialogos-project/dialogos/wiki/Grammars.

[timobaumann]

RHS of a SRGS rule :-) (DialogOS uses SRGS internally, which is then converted to JSGF for Sphinx.)

[alexanderkoller]

That, too, may be worth documenting precisely at some point. As far as I can tell, DialogOS uses a dialect of the ABNF form of SRGS: https://www.w3.org/TR/speech-grammar

Dialect because we do not require, for example, the ABNF self-identifying header.

[alexanderkoller]

Done.