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

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;