Improve error message for Judoc comments without fun/ind definition

[jonaprieto]

Currently, an orphan Judoc comment results in a parsing error. While it's important to flag this issue, the error message could be more informative. Let's improve it to clearly instruct users that a Judoc comment should be accompanied by a declaration.

module A;

--- Nothing more

A.juvix:3:1:
  |
3 | <empty line>
  | ^
unexpected end of input
expecting "--", "---", "{-", "{-#", <identifier>, 
<module definition>, axiom, builtin, import, infix, 
infixl, infixr, newline, open, positive, postfix, terminating, or type