[diff.expr] Update incorrect comment

[Halalaluyafail3]

+arr2 is a constraint violation in C, rather than being valid.

[jensmaurer]

@Halalaluyafail3 , please fix the check failure. (See annotation in diff view.)

[Halalaluyafail3]

Should the above line also be changed to use \Cpp{}? The ^[^%]*[^{"]C++[^"}] pattern in check-source.sh seems to allow C++ at the beginning or end of a line but I do not see why that would be the intent.

[jensmaurer]

Indeed, C23 says

6.5.4.3 Unary arithmetic operators
Constraints
The operand of the unary + or - operator shall have arithmetic type ...

For comparison, C++ says in [expr.unary.op] p7:

The operand of the unary + operator shall be a prvalue of arithmetic, unscoped enumeration, or pointer type...

[jensmaurer]

@tkoeppe , I think this should go into C++26.