doc: document the kernel-doc conventions for kernel hackers

Provide documentation of the kernel-doc documentation conventions oriented
to kernel hackers.

Since I figure that there will be more people reading this
kernel-doc-nano-HOWTO.txt file who are kernel developers focused on the
rest of the kernel, than there will be readers of this file who are
documentation developers extracting that embedded kernel-doc
documentation, I have taken the liberty of making the new section added
here:

  How to format kernel-doc comments

the first section of the kernel-doc-nano-HOWTO.txt file.

This first section is intended to introduce, motivate and provide basic
usage of the kernel-doc mechanism for kernel hackers developing other
portions of the kernel.

Signed-off-by: Paul Jackson <pj@sgi.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>