fix: phpdoc for repeatedfield

[bshaffer]

I have found two issues with PHP protobuf's RepeatedField when using static analyzers:

1. The PHPDoc says that the first argument to offsetSet is an integer, but this value can also be null, in the case when a RepeatedField is being appended to (e.g. $repeatedField[] = $someRepeatedValue). Without marking the first argument as optional, static analyzers think that instances of RepeatedField cannot be appended to.
2. The PHPDoc says that offsetSet accepts object and offsetGet returns object. But object excludes primitive types in PHP such as string int, bool, and array. I believe it should accept primitives, and as a result, in certain circumstances the static analyzers are failing. I've tested this with using the accepted PHP type mixed, which represents any object or primitive. The keyword only exists in PHP 8.0, but this is acceptable documentation for any version of PHP.