documentation: BB_NUMBER_THREADS and PARALLEL_MAKE suggestions

Text that suggests setting the BB_NUMBER_THREADS and
PARALLEL_MAKE variables to speed build time has been updated to
suggest a setting of twice the number of host cores supported.

The 2x number came to light through a discussion on the project
mailing list initiated by Robert P. J. Day.

Reported-by: Robert P. J. Day
(From yocto-docs rev: 1846b41a595b7bd905ca828eaa04112d1533d7a1)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/dev-manual-bsp-appendix.xml

@@ -562,9 +562,10 @@
                 You should also be sure any other variables in which you are interested are set.  
                 Some variables to consider are <filename>BB_NUMBER_THREADS</filename>
                 and <filename>PARALLEL_MAKE</filename>, both of which can greatly reduce your build time 
-                if you are using a multi-threaded development system (e.g. values of 
-                <filename>8</filename> and <filename>j 6</filename>, respectively are optimal 
-                for a development machine that has four available cores).</para></listitem>
+                if your development system supports multiple cores.
+                For development systems that support multiple cores, a good rule of thumb is to set 
+                both the <filename>BB_NUMBER_THREADS</filename> and <filename>PARALLEL_MAKE</filename> 
+                variables to twice the number of cores your system supports.</para></listitem>
             <listitem><para>Update the <filename>bblayers.conf</filename> file so that it includes 
                 the path to your new BSP layer.  
                 In this example you need to include the pathname to <filename>meta-mymachine</filename>.