[dataflow] document flow condition

There's some documentation of this concept at
https://clang.llvm.org/docs/DataFlowAnalysisIntro.html
but it would be nice to have it closer to the code.

I also was laboring under an obvious but wrong mental model that
the flow condition token represented "execution reached this point",
I'd like to explicitly call that out as wrong.

Differential Revision: https://reviews.llvm.org/D154969