Add iterable to the method definitions

[mschop]

Hi,

since PHP 7.1 one can use iterable type hint for everything that is iterable, including array and Traversable.

I know this lib shall also work for older PHP version, but adding iterable to the phpdoc-block would not hurt.

Currently IDE's and other tools throw warnings when I pass an iterable instead of an array or a Traversable.

Suggestion:

Add iterable to all doc-blocks.

Best Regards
Martin