This is as much as a computer science question as an English language one. Bit positions in a numeric word have confusing aspects. We read from left to right, but bits are numbered from right to left. Plus, the lowest order bit is referred to as "bit 0", not "bit 1". So it is not innately clear to English speakers in general whether the "fifth" bit is the 5th from the right or 5th from the left. Even programmers can get confused, especially the newer "kids" who are often taught high level languages without a base knowledge of hardware or even assembly language.
Now to the question itself. I would approach it in one of two ways. If I knew the documentation should be read from start to finish, then I would include a short section at the front with a drawing of a numeric word and its bits, and explain that bit 0 is on the right, bit 1 is next to it, etc. The drawing could show all of this. Then in the rest of the document, just refer to "bit 7" or "bit 23" as if the reader would know what you mean. Another common technical form is that "bit 0" is the 1-bit, "bit 1" is the 2-bit, "bit 4" is the 16-bit, etc. So: The 1 is at bit 5.
If the document is likely to be accessed at random locations, not actually read from start to finish, then for clarity's sake, be clear. Say "the fifth bit from the right" or the "seventh bit from the left", whatever will make it easier for the reader to follow at that particular spot. So: The 1 is at the sixth bit from the right.
htmlcoderexe's comment to this answer got me doing a little more research. The term you are looking for may be bit numbering. Unfortunately this term opens a whole new can of worms - LSB Bit 0 numbering vs MSB Bit 0 numbering. My whole discussion above was based on the former, because I was thinking of a numeric word as a stand-alone number, not at how a specific piece of hardware might store that number. (If nothing else, the Wiki article contains public domain illustrations of bit positions you could use in your document.)