https://www.ietf.org/rfc/ien/ien137.txt
Also more reader-friendly here: https://www.computer.org/csdl/mags/co/1981/10/01667115.pdf
But because most architectures don't provide any way to address individual bits, only bytes, it's entirely up to the observer to decide in which order they want to imagine the bits. When using little-endian, you imagine that the bits are in little-endian order, to be consistent with the bytes, and then everything is nice and consistent.
But isn't that kind of at odds with how shifting works? (i.e. that a left shift moves towards the "bigger" bits and a right shift moves toward the "smaller" ones.) Perhaps for a Hebrew or Arabic speaker this all works out nicely, but for those of us accustomed to progressing from left to right it seems a bit backwards...