doc: Update tool sets documentation for modern FreeBSD

For several operating systems, we give handy package manager one-liners
to install all the requirements for building our documentation.  All
current production FreeBSD releases have a friendly new package manager
a bit like apt/yum, so give a one line command here.  Also, add a brief
note about gmake vs make in the doc subdirectory.

Author: Thomas Munro <thomas.munro@enterprisedb.com>

doc/src/sgml/docguide.sgml

@@ -254,6 +254,19 @@ yum install docbook-dtds docbook-style-dsssl docbook-style-xsl libxslt openjade
     also be of interest.
    </para>
 
+   <para>
+    To install the required packages with <command>pkg</command>, use:
+<programlisting>
+pkg install docbook-sgml docbook-xml docbook-xsl dsssl-docbook-modular libxslt openjade
+</programlisting>
+   </para>
+
+   <para>
+    When building the documentation from the <filename>doc</filename>
+    directory you'll need to use <command>gmake</command>, because the
+    makefile provided is not suitable for FreeBSD's <command>make</command>.
+   </para>
+
    <para>
     More information about the FreeBSD documentation tools can be
     found in the <ulink url="http://www.freebsd.org/doc/en_US.ISO8859-1/books/fdp-primer/tools.html">