printk: comment pr_cont() stating it is only to continue a line

KERN_CONT is nicely commented in kern_levels.h, but pr_cont() is now used
more often, and it lacks the comment stating what it is used for.  It can
be confused as continuing the log level, but that is not its purpose.  Its
purpose is to continue a line that had no newline enclosed.  This should
be documented by pr_cont() as well.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/printk.h

@@ -255,6 +255,11 @@ extern asmlinkage void dump_stack(void) __cold;
 	printk(KERN_NOTICE pr_fmt(fmt), ##__VA_ARGS__)
 #define pr_info(fmt, ...) \
 	printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)
+/*
+ * Like KERN_CONT, pr_cont() should only be used when continuing
+ * a line with no newline ('\n') enclosed. Otherwise it defaults
+ * back to KERN_DEFAULT.
+ */
 #define pr_cont(fmt, ...) \
 	printk(KERN_CONT fmt, ##__VA_ARGS__)