Document that #theme can be an array

@berdir is correct: The value for #theme is usually a string, but it could be an array too. It is not required to be an array.

I am not sure that needs to be documented, as there are few cases where an array is really necessary. (If an array were always expected, Drupal core would require an array by default.)

I checked the 8.x-1.x branch for this project repository, but it is empty. I take the documentation files are not on this project anymore.