Libs(PHP): doc and disable warning in test suite

We've got some test that check that verify() doesn't throw an exception,
but that function doesn't return anything so we can't assert anything
about the outcome.

To hush the warnings in the test suite we have to explicitly tell it "we
don't expect to make any assertions."

Author: svix-onelson

php/tests/WebhookTest.php

@@ -112,6 +112,10 @@ public function testNewTimestampThrowsException()
 
     public function testMultiSigPayloadIsValid()
     {
+        // We're checking that `verify()` doesn't throw an exception.
+        // It doesn't return anything we can assert about.
+        $this->expectNotToPerformAssertions();
+
         $testPayload = new TestPayload(time());
         $sigs = [
             "v1,Ceo5qEr07ixe2NLpvHk3FH9bwy/WavXrAFQ/9tdO6mc=",
@@ -127,6 +131,10 @@ public function testMultiSigPayloadIsValid()
 
     public function testSignatureVerificationWithAndWithoutPrefix()
     {
+        // We're checking that `verify()` doesn't throw an exception.
+        // It doesn't return anything we can assert about.
+        $this->expectNotToPerformAssertions();
+
         $testPayload = new TestPayload(time());
 
         $wh = new \Svix\Webhook($testPayload->secret);