Ezp 22997 - Incorrect code documentation for method ContentService.prototype.loadContent()

Corrected params' default values of method `ContentService.prototype.loadContent()` to any avoid any issues with incorrect concatenation of null-valued params.
Improved code documentation for `ContentService.prototype.loadContent()` method by adding information about params' default values.
nitpick: we usually don't align the `=` like that because if you add a new too long variable you end up with something partly aligned (which is weird) or you need realign everything (which introduces uncessary diff lines)