Add docs to `bytes48_to_G1` why there are no failing tests

[b13decker]

          suggestion: consider adding docs here or on `test_bytes48_to_g1_point` to explain why you're not defining tests for the invalid points.

_Originally posted by @marsella in https://github.com/GaloisInc/ckzg-eip-4844-verification/pull/23#discussion_r1781358236_