[analyzer][UninitializedObjectChecker] Updated comments

Some of the comments are incorrect, imprecise, or simply nonexistent.
Since I have a better grasp on how the analyzer works, it makes sense
to update most of them in a single swoop.

I tried not to flood the code with comments too much, this amount
feels just right to me.

Differential Revision: https://reviews.llvm.org/D51417

llvm-svn: 342215