Add and fix lots of PHPDoc @param, @return and @var tags

• I focused on lines that start with "/// @var" because PHPStorm and

other tools do not realize these are meant to be PHPDoc tags.

• I'm fixing some actual mistakes in some PHPDoc tags.
• I'm changing generic "array" to "string[]" and so on, if possible.
• I'm adding "...|null" when a class property is null by default.
• I'm also removing two class properties that are unused.

Change-Id: Ie114dcc33fe0db58c3c5e283247e14ac8b542fa0