check-code: reject sed ... \\n
This would have caught
142891ab6e89 (fixed by
a5a13eeffc59)
if repcomment didn't make the content of the perl code
opaque to the check.
hideablerevs: expand docstring to warn about possible traps
Sean Farley just wasted multiple hours trying to figure out why his code was
crashing. We update the docstring to make the constraint clearer.