Not exactly true for "update" target - for example, you can "make update"
  either gtk2 or a package dependent on gtk2, in both cases gtk2 will be
  rebuilt if needed.

Perhaps I'm confused, but I thought 'make update' in x replaced the
package x and all packages y such that y depends on x, and not
packages z such that x depends on z.   If that's right, the text above
should arguably be

***  - "make update" to rebuild the package and all depending packages

But on reading Packages.txt I see that update does a recursive 'make
update' for any dependencies that are required to be built.   It seems
that a dependency of x (z above) will only be updated if it is found
to be too old to build, rather than if it is out of date with respect
to pkgsrc.

So really this should perhaps be 

***  - "make update" to rebuild the package, any dependencies for
       which a newer version is necessary, and all depending packages

-- 
        Greg Troxel <gdt@ir.bbn.com>