Add infix operator legal characters to the docs

[marsella]

Cryptol supports definition of custom infix operators, but there are some restrictions on what characters can be used in that definition. They're defined in the lexer; per @yav:

([\!\#\$\%\&\*\+\-\.\/\:\<\=\>\?\@\\\^\|\~] | $unisymbol)+
where unisymbol are things that in unicode are categorized like this:

Char.ConnectorPunctuation                                                     
Char.DashPunctuation                                                               
Char.OtherPunctuation                                                             
Char.MathSymbol                                                                     
Char.CurrencySymbol                                                    
Char.ModifierSymbol                                                                 
Char.OtherSymbol 

We should add documentation somewhere to document what the allowable characters are (and perhaps also the general syntax). I don't see anything about custom-defined infix operators in the book or the reference manual; this would probably fit best in the refman.