Update manual that @start is not required

[Catherine Rodgers]

The manual currently says, "@start must appear once and exactly once
in every grammar." This does not appear to be true: if you leave out a
@start command, then Gazelle assumes you want the top-level
nonterminal to be your @start rule.

I think this is good default behavior. The manual just needs to be
updated to document things accurately.

[Joshua Haberman]

Good catch -- and what's worse, Gazelle isn't currently respecting @start at all!

I fixed the manual, but I still need to fix the compiler to respect @start when it is specified.

Josh