More documentation fixes.

Hi, I've worked a little more on improving the documentation to aid IDEs. In this PR:

 * Updated the IClient and derivates to represent the channel in `IClient::channelStatus()` as a string, as specified by the documentation.
 * Improved PHPDoc for better integration with IDEs.
 * Use the Apache-2.0 SPDX identifier for the license (see the commit message for more info). This has no impact on the licensing of this software, it just standardizes it and therefore makes it machine-readable.

Thanks, Jacob