pdl: Update endianness documentation

Reverse the bit order to use MSB -> LSB,
as this is more natural for most readers.

Test: none
(cherry picked from https://android-review.googlesource.com/q/commit:91b6ec3d0711991d6f02ca954948f65bb7e416c8)
Merged-In: I373094a51ff466d46a0f9952326c562bcff0a6ae
Change-Id: I373094a51ff466d46a0f9952326c562bcff0a6ae