credentials validation is not consistent across libraries.

[audas]

This should be made clear to users.

For example in order to use Speech to text users can use the contents of a keyFile as "credentials

new SpeechClient(['credentials' => $credentials]);

HOWEVER this fails with STORAGE and users must use a keyPath

    $storage = new StorageClient([
        'keyFilePath' => 'path_to_.json',
        'projectId' => 'projectID'
    ]);

This is in my view a terrible programmatic anomaly and there should be uniform approach to validation so it should be made very clear - and since there is almost ZERO documentation on authorization which is instead farmed out via links which lead to no where, or simply do not work, it is worth while addressing this failure in the documentation.

[danoscarmike]

@frankyn @bshaffer for vis