Improve ExUnit seed docs (#10962)

Make it more obvious what a seed of 0 does.

Author: axelson

lib/ex_unit/lib/ex_unit.ex

@@ -274,7 +274,9 @@ defmodule ExUnit do
     * `:seed` - an integer seed value to randomize the test suite. This seed
       is also mixed with the test module and name to create a new unique seed
       on every test, which is automatically fed into the `:rand` module. This
-      provides randomness between tests, but predictable and reproducible results;
+      provides randomness between tests, but predictable and reproducible
+      results. A `:seed` of `0` will disable randomization and the tests in each
+      file will always run in the order that they were defined in;
 
     * `:slowest` - prints timing information for the N slowest tests. Running
       ExUnit with slow test reporting automatically runs in `trace` mode. It

lib/mix/lib/mix/tasks/test.ex

@@ -159,7 +159,8 @@ defmodule Mix.Tasks.Test do
     * `--raise` - raises if the test suite failed
 
     * `--seed` - seeds the random number generator used to randomize the order of tests;
-      `--seed 0` disables randomization
+      `--seed 0` disables randomization so the tests in a single file will always be ran
+      in the same order they were defined in
 
     * `--slowest` - prints timing information for the N slowest tests.
       Automatically sets `--trace` and `--preload-modules`