Add more docs to the cli.h file in SPIRV-Tools fuzzer.
Change-Id: I327c0f6919c07724ecb471655fb1a284fc6c43ae
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56065
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>