Fix and improve language reference

[treiher]

Errors

• Add missing return type in syntax of function_declaration
• Replace when by if in syntax of comprehension
• Add name after quantifier in syntax of quantified_expression

Improvements

• Explicitly mention that Channel is a special type only usable for declaring channels
• Make semantics of the state actions more precise, (e.g., what is the intent of the arguments to the Reset attribute, and what is the effect of 'Read)