docs: kernel-doc.rst: add documentation about man pages

kernel-doc-nano-HOWTO.txt has a chapter about man pages
production. While we don't have a working  "make manpages"
target, add it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/kernel-doc.rst

@@ -452,3 +452,37 @@ file.
 
 Data structures visible in kernel include files should also be documented using
 kernel-doc formatted comments.
+
+How to use kernel-doc to generate man pages
+-------------------------------------------
+
+If you just want to use kernel-doc to generate man pages you can do this
+from the Kernel git tree::
+
+  $ scripts/kernel-doc -man $(git grep -l '/\*\*' |grep -v Documentation/) | ./split-man.pl /tmp/man
+
+Using the small ``split-man.pl`` script below::
+
+
+  #!/usr/bin/perl
+
+  if ($#ARGV < 0) {
+     die "where do I put the results?\n";
+  }
+
+  mkdir $ARGV[0],0777;
+  $state = 0;
+  while (<STDIN>) {
+      if (/^\.TH \"[^\"]*\" 9 \"([^\"]*)\"/) {
+	if ($state == 1) { close OUT }
+	$state = 1;
+	$fn = "$ARGV[0]/$1.9";
+	print STDERR "Creating $fn\n";
+	open OUT, ">$fn" or die "can't open $fn: $!\n";
+	print OUT $_;
+      } elsif ($state != 0) {
+	print OUT $_;
+      }
+  }
+
+  close OUT;