Fix grammar around function declaration

[Issue 10235](https://issues.dlang.org/show_bug.cgi?id=10235) - Grammar does not contain a rule for function declarations
[Issue 12579](https://issues.dlang.org/show_bug.cgi?id=12579) - DMD rejects valid function literal
[Issue 13523](https://issues.dlang.org/show_bug.cgi?id=13523) - Auto function declaration does not match any grammar rule

And I moved function-related grammars, `Parameters`, `FunctionAttribute`, `MemberFunctionAttribute`, etc into the `function.html` page.
SpecialKeyword is already present in the PrimaryExpression rule. I think we can remove this and let the AssignExpression rule handle special keywords.