[DOC] State the precision of Process.times as platform-defined

Remove the bad example that can lead to misunderstanding as if this
precision is defined in Ruby.

process.c

@@ -7836,6 +7836,7 @@ get_clk_tck(void)
  *    Process.times
  *    # => #<struct Process::Tms utime=55.122118, stime=35.533068, cutime=0.0, cstime=0.002846>
  *
+ *  The precision is platform-defined.
  */
 
 VALUE

spec/ruby/core/process/times_spec.rb

@@ -16,31 +16,4 @@ describe "Process.times" do
       Process.times.utime.should > user
     end
   end
-
-  # TODO: The precision of `getrusage` depends on platforms (OpenBSD
-  # seems not supporting under-milliseconds in fact); this example is
-  # very questionable as an example of Ruby, and it just repeats the
-  # guard condition.
-  guard -> do
-    1000.times.any? do
-      # If getrusage has precision beyond milliseconds, there will be
-      # very likely at least one non-zero microsecond results when
-      # repeating enough.
-      time = Process.clock_gettime(:GETRUSAGE_BASED_CLOCK_PROCESS_CPUTIME_ID, :nanosecond)
-      not (time % 1_000_000) == 0
-    end
-  rescue Errno::EINVAL
-    false
-  end do
-    it "uses getrusage when available to improve precision beyond milliseconds" do
-      max = 10_000
-
-      found = (max * 100).times.find do
-        time = Process.times.utime
-        !('%.6f' % time).end_with?('000')
-      end
-
-      found.should_not == nil
-    end
-  end
 end