Add comments explaining @runInSeparateProcess usage

The output buffer callback calls header() which requires
a separate process since PHPUnit has already started output.

Author: adamsilverstein

tests/phpunit/tests/media/wpCrossOriginIsolation.php

@@ -64,6 +64,10 @@ public function test_returns_early_when_no_screen() {
 	}
 
 	/**
+	 * This test must run in a separate process because the output buffer
+	 * callback sends HTTP headers via header(), which would fail in the
+	 * main PHPUnit process where output has already started.
+	 *
 	 * @ticket 64766
 	 *
 	 * @runInSeparateProcess
@@ -121,6 +125,10 @@ public function test_does_not_start_output_buffer_for_safari() {
 	}
 
 	/**
+	 * This test must run in a separate process because the output buffer
+	 * callback sends HTTP headers via header(), which would fail in the
+	 * main PHPUnit process where output has already started.
+	 *
 	 * @ticket 64766
 	 *
 	 * @runInSeparateProcess