Miscellaneous small changes. Includes reformatting white space in some
of the example HTML and DocBook fragments, which are significant.

Explain where to find the FreeBSD DTD that extends DocBook.

Add a section explaining about books and articles, and the differences
between them.

Added an example of <procedure>.

Be consistent with the white space inside <programlisting>. Make sure
that content starts immediately after the opening tag. Shouldn't make
a great deal of difference, but we should practice what we preach.

Remove the information about <informalexample>, as it seems to be
redundant, and we don't use it anymore.

Correct an error ("]]", should be "]") in the example of including the manual
page reference entities.