This is a little more complicated than adding a warning in the right place. The parser will have to mark the conditional expression in an 'until' with a new flag, as a loop condition expression that is not braindead, just so the evaluator can see if the & or | operators are used in the conditional expression and issue a new warning message. Something like "warning: Matlab-style short-circuit operation not performed for operator |"?

I'm OK with adding a warning if the conditions for Matlab short-circuiting occur in the condition of an UNTIL statement, but I don't want to change the behavior of UNTIL statements to do that kind of short circuiting.  It's not needed for compatibility since Matlab doesn't have this statement.

There is another place in the manual also: Section 8.5.2:

matlab has special behavior that allows the operators ‘&’ and ‘|’ to short-circuit when
used in the truth expression for if and while statements. Octave behaves the same way for
compatibility, however, the use of the ‘&’ and ‘|’ operators in this way is strongly discouraged
and a warning will be issued. Instead, you should use the ‘&&’ and ‘||’ operators that always
have short-circuit behavior.

One could mention until also:
As matlab does not support until-statement,
in octave, & and | behave regularly in until-conditions.

Also a reference to the warning would be fine.