fix(s3demo.php): Rename expectedBucket to expectedBucketName

Should clear up some confusion for integrators.
feat(s3demo-cors.php): Rename S3 PHP demo to indicate CORS use

Should help integrators be less confused.

feat(php/s3/s3demo.php): Non-CORS example

Lessen the user confusion.
fix(php/s3/s3demo.php, s3demo-cors.php): Document $expectedMaxSize

Confusing users. By setting this var to `null` we assume that no size
validation is going on. If it has a value, then it should match
the value set for the validation.sizeLimit option property.