scripts/kernel-doc: Do not process backslash lines in comments

To prevent this, do the pre-processing only for lines which are no
comments, e.g. do not start with ' *'.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240122093152.22536-3-anna-maria@linutronix.de