docs: scripts/kernel-doc: Detect absence of FILE arg

Currently, when there is no FILE argument following a switch such
as -man, -rst, or -none, kernel-doc exits with a warning from perl
(long msg folded):

    Use of uninitialized value $ARGV[0] in pattern match (m//)
    at ./scripts/kernel-doc line 438.

, which is unhelpful.

Improve the behavior by adding a check at the bottom of parsing
loop.
If the argument is absent, display help text and exit with
the code of 1 (via usage()).
Signed-off-by: NAkira Yokosawa <akiyks@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/7b136049-a3ba-0eb5-8717-364d773ff914@gmail.com
[jc: reworked to fix conflict with pod patches]
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

scripts/kernel-doc

@@ -352,6 +352,15 @@ while ($ARGV[0] =~ m/^--?(.*)/) {
 	    -output => \*STDERR,
 	    );
     }
+    if ($#ARGV < 0){
+	pod2usage(
+	    -message => "FILE argument missing\n",
+	    -exitval => 1,
+	    -verbose => 99,
+	    -sections => 'SYNOPSIS',
+	    -output => \*STDERR,
+	    );
+    }
 }
 
 # continue execution near EOF;