scripts: kernel-doc: Replace the usage function

Aim: unified POD, user more satisfied, script better structured

You can see the results with:

$ scripts/kernel-doc -help
Signed-off-by: NTomasz Warniełło <tomasz.warniello@gmail.com>
Tested-by: NRandy Dunlap <rdunlap@infradead.org>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Disliked-by: NAkira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/20220218181628.1411551-10-tomasz.warniello@gmail.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

scripts/kernel-doc

@@ -79,15 +79,6 @@ See Documentation/doc-guide/kernel-doc.rst for the documentation comment syntax.
 # 25/07/2012 - Added support for HTML5
 # -- Dan Luedtke <mail@danrl.de>
 
-sub usage {
-    my $message = <<"EOF";
-Usage: $0 [OPTION ...] FILE ...
-
-EOF
-    print $message;
-    exit 1;
-}
-
 #
 # format of comments.
 # In the following table, (...)? signifies optional structure.
@@ -468,7 +459,7 @@ while ($ARGV[0] =~ m/^--?(.*)/) {
     } elsif ($cmd eq "Werror") {
 	$Werror = 1;
     } elsif (($cmd eq "h") || ($cmd eq "help")) {
-	usage();
+		pod2usage(-exitval => 0, -verbose => 2);
     } elsif ($cmd eq 'no-doc-sections') {
 	    $no_doc_sections = 1;
     } elsif ($cmd eq 'enable-lineno') {