[review request] New config.5 manual page

[John]

Dealing with maxusers can be a bit confusing depending upon
the approach taken.  You may want to add a bit of additional
information:

     maxusers number
                  This optional directive is used to configure the size of
                  some kernel data structures.  number should be an integer
                  between 2 and 512, or can be 0.  From FreeBSD 4.5 onwards,
                  the kernel will automatically size its tables according to
                  the available physical memory if maxusers is not specified
                  or is specified to be 0.  As explained in tuning(7), this
                  tunable can also be set on a running kernel using sysctl(8).

<added>
                  When maxusers is not specified and thus automatically
                  sized, the default maximum is 384. To increase maxusers
                  beyond 384 requires either a config entry or boot time
                  override. As noted above, the config entry for maxusers
                  is limited to 512. If a maxusers of 1024 is desired, an
                  entry in /boot/loader.conf is required.
</added>

   If the maxusers can be specified in multiple locations, the limits
placed on the value should be similar/identical. Any differences in the
way the value is treated should be well documented (or fixed).  As we
move into larger memory systems our current limitations need to be either
removed or increased.
                  
   Hope this helps.

-John