`cfg-parser` BNF inaccurate wrt implementation

[jasonhemann]

IIUC, the syntax block for the cfg-parser says that it takes 0 or more clauses, where a clause is any one of those 6 options. It seems that cfg-parser requires at least one clause, and necessarily a grammar clause at that, to even generate valid syntax. Furthermore, I believe the grammar form claims it admits parse rules for 0 or more non-terminals, but the implementation requires at least one parse rule, and I believe that each RHS actually requires one or more grammar-ids, rather than 0 as stated.

See:

parser-tools/parser-tools-doc/parser-tools/parser-tools.scrbl

Lines 722 to 734 in 4afd434

	@defform/subs[#:literals (grammar tokens start end precs src-pos
	suppress expected-SR-conflicts expected-RR-conflicts
	debug yacc-output prec)
	(cfg-parser clause ...)
	([clause (grammar (non-terminal-id
	((grammar-id ...) maybe-prec expr)
	...)
	...)
	(tokens group-id ...)
	(start non-terminal-id ...)
	(end token-id ...)
	(@#,racketidfont{error} expr)
	(src-pos)])]{

*IIUC