[AST / Documentation] Unclear specification for the barrier instruction

[nelimee]

The issue

In the AST node for the barrier statement, the arguments following the barrier keyword are Expressions.
I think it should rather be similar to the AST node for the reset statement in which the arguments are registers or qubits (Union[IndexedIdentifier, Identifier]).

The corresponding documentation seems to agree:

The syntax is barrier qregs|qubits;

As a side note, this particular part of the documentation seems outdated, citing OpenQASM 2 instead of OpenQASM 3:

The barrier instruction of OpenQASM 2 prevents [...]

Expected enhancement?

• Make the AST node more specific (List[Union[IndexedIdentifier, Identifier]] instead of List[Expression]) or, if the barrier statement should accept more than quantum registers, make this explicit in the documentation.
• Also, update the documentation to at least cite OpenQASM 3 instead of OpenQASM 2.