pdl: Explain possible canonical test failures

Test: atest pdl_tests pdl_rust_generator_tests_{le,be}
(cherry picked from https://android-review.googlesource.com/q/commit:a1ee89e3864cd7ca6e264fb450d5e002f54ff8e5)
Merged-In: I6366b88f2b4683baf21e31519ccd981b6e81a380
Change-Id: I6366b88f2b4683baf21e31519ccd981b6e81a380