Adding comments and DCHECKS in the validation logic of configuring QuicStream's flow controller.
Adding comments and DCHECKS in the validation logic of configuring QuicStream's flow controller.

No behavior change. not protected.

PiperOrigin-RevId: 326348719
Change-Id: Id21245f96e7561e3d4ddd88e7bf132eb2ff49e40

google/quiche,google/quiche,google/quiche,google/quiche